Aging Methods and Protocols (Methods in Molecular Medicine)

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TM M E T H O D S I N M O L E C U L A R M E D I C I N E TM

Aging Methods and Protocols Edited by

Yvonne A. Barnett Christopher R. Barnett

Humana Press

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1 Understanding Aging Bernard L. Strehler 1. Background Enormous advances in our understanding of human aging have occurred during the last 50 yr. From the late 19th to the mid-20th centuries only four comprehensive and important sources of information were available: 1. August Weismann’s book entitled Essays on Heredity and Kindred Biological Problems (the first of these essays dealt with The Duration of Life; 1). Weissmann states (p. 10) “In the first place in regulating the length of life, the advantage to the species, and not to the individual, is alone of any importance. This must be obvious to any one who has once thoroughly thought out the process of natural selection…”. 2. A highly systematized second early source of information on aging was the collection of essays edited by Cowdry and published in 1938. This 900+ page volume contains 34 chapters and was appropriately called Problems of Aging. 3. At about the same time Raymond Pearl published his book on aging (2). Pearl believed that aging was the indirect result of cell specialization and that only the germ line was resistant to aging. Unfortunately Pearl died in the late 1930s and is largely remembered now for having been the founding editor of Quarterly Review of Biology while he was at the Johns Hopkins University, this author’s alma mater. 4. Alexis Carrel wrote a monumental scientific and philosophical book, Man, the Unknown (3). Carrel believed that he had demonstrated that vertebrate cells could be kept in culture and live indefinitely, a conclusion challenged by others (more on this later).

Probably the most useful of all the more recent books published on aging was Alex Comfort’s The Biology of Senescence (4), which supplied much of the source information that this author used in writing Time, Cells and Aging (5–7; I am most grateful to Dr. Christine Gilbert, of Cyprus, for her efforts in From: Methods in Molecular Medicine, Vol. 38: Aging Methods and Protocols Edited by: Y. A. Barnett and C. R. Barnett © Humana Press Inc., Totowa, NJ

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the revision of the third edition of Time, Cells and Aging, and for the most stimulating discussions we have had over the years). The extremely useful and thoroughly documented book called Developmental Physiology and Aging by Paul Timeras (8) is a fine source of critical appraisals of the science in both areas. Many of the more recent books on aging are cited later. The success of my own journal (Mechanisms of Ageing and Development) is largely due to the work of our excellent editorial board and to the careful work and prodding of my dear wife, Theodora Penn Strehler, who passed away on 12 February, 1998. This chapter is dedicated to her living memory and the love she gave to me for 50 years of marriage and joy and sadness — and the kindness she showed to all who knew her. Requiescat in pacem. 2. Overview of a Systematic Approach My own synthesis and analysis of the nature and causes of aging were presented in a book called Time, Cells and Aging. To use terms consistently in discussing aging, a set of four properties that all aging processes must meet are defined in that book: 1. Aging is a process; i.e., it does not occur suddenly, but rather is the result of very many individual events. 2. The results of aging are deleterious in the sense that they decrease the ability of an individual to survive as he or she ages. 3. Aging is universal within a species. However, aging may not occur in every species. Thus, certain “accidents” such as those that result from a specific infection are not part of the aging process. 4. Aging is intrinsic to the living system in which it occurs (i.e., it reflects the qualities of DNA, RNA, and other structures or organelles that were inherited from the parental generation).

The central thesis presented in Time, Cells and Aging is that the possible causes of aging can be divided into: 1. Those that are built into the system as specific DNA or RNA coding (or catalytic) sequences, and 2. Those that are the result of controllable or uncontrollable environmental factors including radiation, nutrition, and lifestyle.

Two key phenomena are shown by aging animals: 1. The probability of a human dying doubles about every 8 yr, a fact that was first discovered by an English Insurance Actuary by the name of Benjamin Gompertz about 165 yr ago (9). Thus, the following equation, derived from Gompertz’s work, accurately describes the probability of dying as a function of age in a particular environment: R = k + R0eat where, R(ate) of death at any age equals the probability of dying at age 0 multiplied by an age-dependent factor that is equal

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to e raised to the a times t power, where a is a function of the doubling time and t is the age attained. A better fit to observed mortality rates is given by adding a constant (k) (which largely reflects environmental factors). If one plots log R against t(age) one obtains a remarkably precise straight line, usually between ages 30 and 90. A Gompertz curve is obtained for the mortality rate vs age for a variety of animals—humans, horses, rats, mice, and even Drosophila melanogaster, a much studied insect. 2. A second general fact or law is provided by my own summary and analysis of the pioneering quantitative work of Nathan Shock on maximum functional ability of various body systems’ ability to do work as humans age. Shock’s studies (on humans) implied to me that after maturity is reached the following equation describes a multitude of maximum work capacity of various body parts: Wmax = Wmax(30) (1 – Bt) where B varies from about 0.003 per yr to almost 0.01 per yr— depending on the system whose maximal function is being measured. For example, maximum nerve conduction velocity declines by about 0.003 per yr (10) and vital capacity as well as maximum breathing capacity declines by about 1% per yr (11).

The Gompertz and Shock equations pose the following puzzling and key question: “How can a linearly declining ability in various functions cause a logarithmic increase in our chances of dying as we age ?” A probable answer to this question was provided by this author in collaboration with Prof. Albert Mildvan (12–14). Our theory made two assumptions. The first of these is that the equation derived from Shock’s work (that the maximum work capacity of a variety of body systems declines linearly after maturity is reached) is valid. This, as shown earlier, is the very simple equation: Wmax = Wmax(30) (1 – Bt), where Wmax is the maximum ability to do work at age t, Wmax(30) is the maximum ability to do work at age 30, where B is the fraction of function lost per yr, and t is the age in years. Of course B varies from species to species and the t term is some small fraction of the maximum longevity of a species. The second assumption is that the energy distribution of challenges to survival is very similar to the kinetic energy distribution of atoms and molecules as defined in the Maxwell–Boltzmann equation. This equation or law defines how kinetic energy is distributed in a collection of atoms or molecules at a specific temperature (where temperature is defined as the average kinetic energy and is equal to KE = 0.5 mv2). This distribution has a maximum value near the average kinetic energy of the particles in the system. But higher and higher energies are generated through random successive multiple collisions between particles. The reason that this is possible is easily understood through an analogy in which the particles are seen as billiard balls. Consider the case when one of two spherical billiard balls can absorb momentum from another such sphere. This happens in billiards when one ball strikes the second ball squarely. In that case, the moving billiard ball stops and the formerly stationary

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one moves off at about 45° from the direction in which the first one was moving. The law of conservation of momentum is mv = K for any two colliding structures. Because the balls are not perfectly elastic some heat will be generated during the collision, but this is a very small fraction of the total momentum and kinetic energy of the two particles. This is evident from the fact that one cannot feel a warming of either of the billiard balls after such a collision and the fact that the ball that is struck moves at about the same velocity that the first ball had before the two balls collided. Now consider the special case where two such billiard balls are traveling at right angles to each other when they collide and that the collision between them is “on center” so that one of the balls stops dead in its tracks and the other ball moves off at a 45° angle at a speed that conserves total momentum. (That is, the moving ball is now moving along the line that defined the center of gravity of the two balls as they were moving before they collided.) If momentum the two balls is conserved (the momenta are added) then the speed of the struck moving ball should be twice that which both of the balls had before they collided. There is no obvious reason why momentum is not conserved in this manner. But the kinetic energy (1/2)mv2 of the moving ball will be much greater than the sum of the kinetic energies they had before collision. (In fact the total kinetic energy of the two balls moving at the same velocity before they collided is two times as great after they collide than it was before this special kind of collision happened!) This is a most surprising seeming “violation” of the Law of Conservation of Energy. It would seem to follow from this that certain kinds of very improbable collisions result in an increase in the kinetic energy of the pair of balls. This seems almost obvious from the fact that the kinetic energies of atoms or molecules is not equal among atoms or molecules in a closed system. Instead, it follows the Maxwell–Boltzmann distribution. Where does this energy come from? Perhaps from the Einsteinian conversion of mass to energy. Thus it appears that if one constructs a device in which collisions of the non-random kind described previously took place one should be able to get more energy out of the system than one puts in— essentially because the structure of such a machine minimizes the entropy of collisions by causing only certain very rare collisions to take place. I have spent many months testing this revolutionary theory, but the results produced from my “Perpetual Motion Machine” have failed to demonstrate any such gain in kinetic energy. There appears to be no other explanation for the distribution of kinetic energy among atoms and molecules than the kind of collisions discussed here! It’s unfortunate that it doesn’t work at the macro level. In any event, if a small probability exists that improbable collisions, such as discussed previously, are rare and cause an increase in momentum of one of the balls or atoms then the probability that a series of similar collisions that increase momentum of particular atom or molecule will give that atom or molecule greater and

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greater energy will decrease very rapidly as the number of such improbable events increases. In fact, the number of such combined events will decrease logarithmically as the energy possessed by such an atom or molecule increases linearly. Such a decreasing exponential is part of the classical form of the Maxwell–Boltzmann equation—and defines the number of atoms with momenta greater than some particular high value. In fact, the distribution of momentum is described by a symmetrical bell-shaped curve (a Maxwellian curve) whereas the distribution of energy follows the Maxwell–Boltzmann curve. To return to the Gompertz equation as it applies to the probability of dying vs age, Mildvan and I postulated that the energy distribution of challenges to living systems is very similar to the Maxwell–Boltzmann distribution. For example. obviously one knows that small challenges such as cutting a finger or tripping or stumbling are very frequent compared to the chance of falling down the stairs, being hit by a speeding automobile, or experiencing an airplane crash. Similarly, the frequency of coming down with a very serious diseases (infections by a new influenza virus, blood clots in the coronary arteries or key arteries in the brain, aortic aneurysms, cancer) is much rarer than is coming down with a minor infection (e.g., a cold or acne) or bumping one’s shin against a coffee table. It may have been that the “Sidney” flu somehow was exported from Hong Kong to Australia by a “carrier” passenger in an airplane and thence to the Uunited States via another carrier who gave it to someone who infected my great grandson, who in turn infected our entire family at Christmas time, 1997 and led to my sadness at losing the person, Theodora (my wife), I had deeply loved and enjoyed for 50 years. The separate events leading to this personal tragedy were each improbable, but they resulted in a very large challenge that one of us was unable to overcome! This illustrates the principle that it takes many unlikely events to lead to a major challenge to humans—or to molecules. The theory of absolute reaction rates states that R = C(kt/h)e–(F*/RT), where F* is the free energy of activation of a reaction. The free energy of activation is in turn defined as the amount of energy needed to break a bond that must be broken in order for a chemical reaction to occur. Of course the free energy needed is derived from multiple collisions and the number of particles that possess a given excess energy equal to that required for a given reaction to occur increases as a function of the absolute temperature. Note that the RT (gas constant times absolute temperature) leads to an exponentially decreasing rate of reaction as T (absolute temperature) is lowered linearly because the T term is in the dividend of the negative exponential term e–(F*/RT). If one plots the log of the rate against 1/T one obtains a straight line whose slope is a measure of the minimum amount of energy (T*) required to cause a reaction to happen. Such a plot is called an Arrhenius plot. Therefore, if one defines the events that

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lead to possible death similarly and takes into account the linear decline in the body’s ability to resist challenges (through the expenditure of the right kind of energy in a particular system or systems) decreases linearly as we age, one obtains the Gompertz equation. Thus, the Gompertz equation results from the logarithmic distribution of size of challenges we encounter interacting with linear loss of functions of various kinds during aging observed by Shock. 3. Ten Key Experimental Questions—Plus Some Answers Although several hundred specific questions or theories regarding the source(s) of aging in humans and other nucleated species (eukaryotes) are possible, only 10 of the most carefully examined “theories” are highlighted here. Space does not permit a complete discussion of each of these questions. 1. How does the temperature of the body affect the rate of aging? The activation energy of a particular chemical reaction is the amount of energy that is derived from accidental collisions among atoms or molecules to break the bonds needed for the reaction to occur. If the reaction is a catalyzed one then the activation energy is about 10–20 kcal/mol. By contrast, if the reaction is not catalyzed the energy required is that which will break a bond in a reacting substance. Covalent bonds require between 75 and 130 kcal to be broken, whereas in the presence of an appropriate catalyst the bond is weakened by its combination with the catalyst so that it only takes 6–20 kcal to break it. If one plots the log of the rate of a reaction against the reciprocal of the absolute temperature one often obtains a remarkably straight line. Such a plot is called an Arrhenius plot (after the man who discovered it). The slope of the straight line obtained in such a plot will generally be high (50–200 kcal for uncatalysed reactions and 6–19 kcal for catalyzed ones. In order to calculate the activation energy of aging I plotted my own results on the effects of temperature in Drosophila life-spans (15,16) together with those of Loeb and Northrup (17,18) and others and found the activation energy to be between 15 and 19 kcal. Thus, in the cold-blooded animal, Drosophila (a fruit fly), the rate of aging appears to be determined by a catalyzed reaction or possibly by the effects of temperature on the rates of production and destruction of harmful substances such as OH radicals that attack DNA and other cell parts. It is known that trout live much longer in cold lakes than in warmer ones but no quantitative studies of their longevities at a variety of temperatures have, to my knowledge, been made. Because mammals operate at essentially constant body temperatures, it is not an easy matter to study the effect of body temperatures on humans or similar mammals. One might find a correlation between the body temperatures of the descendants of centenarians and the descendants of shorter lived persons, but such a study is unlikely to be funded (as I know from personal experience!).

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2. Are changes in connective tissue a key cause of aging? There is no doubt the age-related alterations to the structure and therefore biological properties of connective tissues can lead to cosmetic through to pathological changes in vivo. The onset of such pathologies may in some instances increase the chances of death. It is widely recognized that changes in the elasticity of skin (less elasticity) as we grow older occurs in humans. If one pinches the skin on the back of the hand and pulls up on it, it returns to its original shape (flat) in a short time, about 1 s for young persons and about 3 s or more for older skin. This change is primarily due to the attrition of the elastic fibers that are present in the dermis. If the skin is exposed during early life to large amounts of ultraviolet radiation such as that in sunlight, some of the collagen is converted into a fiber that resembles elastin. This transformation leads to the uneven contraction of the skin, that is, wrinkles are formed. The collagen in the skin and elsewhere in the body becomes less plastic as it matures (for a discussion of the chemical processes underlying these maturity changes please see 19–23). Alteration in the physical properties of the elastic tissue found in blood vessels can lead to changes in blood pressure in vivo. There are many examples of pathologies that result from age-related alterations to connective tissues. Particularly in fair-skinned persons, exposure to ultraviolet light can lead to damage of skin cells and may lead to basal cell and squamous cell cancers (both of which are relatively easily treated) and even melanomas (difficult to treat successfully if not diagnosed at very early stages). Alterations to the structure of bone can lead to osteoporosis. Physical changes to the cartilage in joints can lead to the onset of osteoarthritis. 3. Does a significant fraction of the mitochondria of old mammals suffer from defects, either in DNA or in other key components? The mitochondria we possess are all derived from our mother’s egg, as are various other materials such as particular RNA molecules. Mitochondria are the cell factories in which the energy provided when food is oxidized is converted into the unstable molecule called ATP. ATP is used to contract muscles, to pump ions across neural membranes, and is used to manufacture proteins and RNAs. The production of ATP can be assayed (24–26; John Totter and I (at the Oak Ridge National Laboratory in 1951) developed an assay for ATP using McElroy’s reaction (24) that is able to measure a billionth of a gram of ATP (1 millionth of a milligram). This method has been widely used in various biological and biomedical studies but the description of the method was published so many years ago (1951–52) that it is no longer associated with our names. In my laboratory we used this assay to study the production of ATP by mitochon-

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dria obtained from animals of different ages. We found no differences between mitochondria from 8-mo-old rat hearts and 24-mo-old rat hearts, using α-ketoglutaric acid as substrate. Later it was reported that some mitochondria from old animals oxidize different substrates such as succinate less efficiently than do mitochondria derived from young animals. Later in this book Miquel et al. summarize the literature, including much of their own work, on various morphological and functional changes that accumulate with age in mitochondria. These changes are thought to result from an accumulation of various types of mutations in the mitochondrial genome (much of which codes for polypeptides involved in Complex I and II of the respiratory redox chain) that result from primarily reactive oxygen species damage to the mitochondrial genome that is poorly, if at all, repaired. Turnbull et al. present two chapters later in this book on the analysis of mitochondrial DNA mutations. Such an age-related decrease in mitochondrial function has been proposed to lead to the bioenergetic decline of cells and tissues and so contribute to the aging process (27). 4. Is a limitation in the number of divisions a body cell can undergo (in cell culture) a significant cause of aging? Alexis Carrel reported (3) that he was able to keep an embryonic chicken heart alive for more than 22 yr. This is, of course, much longer than chickens usually live and Carrel concluded that regular supplements of the growth medium with embryo extracts would keep these cultures alive for very long times, perhaps indefinitely. To quote from p. 173 of the Carrel book, “If by an appropriate technique, their volume is prevented from increasing, they never grow old.” Colonies obtained from a heart fragment removed in January 1912, from a chick embryo, are growing as actively today as 23 yr ago. In fact, are they immortal? Maybe so. For many individuals, including myself at about 13 yr of age, these findings were very exciting. Perhaps man would eventually be able to conquer his oldest enemy, aging. It was at about that time that I decided on a career in aging research. In 1965 my good friend Leonard Hayflick reported some research he and a colleague (Moorhouse) had carried out that appeared to be contrary to what the renaissance man, Carrel, had concluded (28). Hayflick found that human fibroblasts in a culture medium could go through only about 50 doublings, after which the cells died or stopped dividing (now known as replicative senescence) or both. Hayflick’s data have been confirmed by many persons, including this author, who with Robert Hay (29) carried out similar experiments on chicken fibroblasts that were only capable of about 20 doublings. However, because a new layer of skin cells is produced about every 4 d (about 90 doublings per yr and 9000 doublings in a 100-yr lifetime), and because red blood cells are produced by the millions every 120 d and because the crypt cells in the lining of

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the intestine give rise to the entire lining of the cleft in which the crypt cells lie, it seemed to me unreasonable that the Hayflick limit applies to normal cells in the body. In the case of skin cells Hayflick countered with the idea that if each of the progenitor cells in the skin could divide only 50 times, then the reason might be that cells moved out of the dividing cell structure (the one cell thick, basal cell layer) that gives rise to the epidermis after they had gone through 40 or 50 doublings. This seemed a reasonable and possibly correct theory, so (with the help of my late wife), we showed that the cells did not leave the basal layer two or four or eight cells at a time, but rather the daughter cells of cells labeled with tritiated thymine moved out of the basal layer randomly (the reader is encouraged to read pp. 37–55 of the third edition of Time, Cells and Aging for further discussion in this regard). Such a finding may cast strong doubt on the relevance of in vitro clonal “aging” to the debilities of old age. I offer one possibility that may account for the apparent contradiction between the findings of Carrel on one hand and of Hayflick on the other. The antibiotics routinely used during the “fibroblast cloning” experiments (and other experiments performed since on the phenomenon of replicative senescence) might in themselves cause a decrease in the number of divisions possible. Carrel was unable to use antibiotics in his studies because they were not yet discovered or manufactured when he carried out his 22-yr experiment on chick heart viability. Hayflick states in his recent book that he has evidence that Carrel’s embryo extract supplements contained living cells and that this is why the tissues Carrel studied remained alive for times greater than the lifetime of a chicken. Carrel had to use very careful means to replace his media every so often over a period of 20 yr. Besides, Carrel did not allow his organ cultures to grow, so cell division was either absent or cells possibly present in the embryo extracts he added were able to differentiate into replacement cells for heart tissues. Because the heart is a syncytium of cells, it is difficult to imagine how a steady state of replacement of old cells by cells possibly present in the embryo extract could take place, particularly within the center of the organ culture! This logic argues for the validity of Carrel’s reports. Moreover, fibroblasts are quite different from myoblasts and do not form syncytia. In very recent times a popular proposal has been that telomeres, the sequences of noncoding DNA located at the end of chromosomes, shorten each time a normal cell divides and that in some way this shortening “counts” the number of cell divisions that a cell population has experienced, perhaps owing to the loss of essential genes that have critical functions for cell viability (30,31). What is not clear is how the documented process of replicative senescence in vivo leads to the development of physiological malfunction and the onset of age-related pathologies in vivo. Changes in the expression of a number of gene functions, including increases in the expression of genes coding for

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growth factors and extracellular matrix components, have been found by studying cells in replicative senescence in vitro. Researchers have been able to detect relatively small numbers of senescent fibroblasts and epithelial cells in older animals and human tissues in vivo using β-galactosidase staining (pH > 6). They have postulated that even such small numbers of cells, exuding various entities because of activated genes etc., might be sufficient to alter tissue homeostasis and so lead to physiological effects. This suggestion has yet to be proven and the role of replicative senescence in aging remains an area of intense research activity. 5. Are errors in the transcription and/or translation of DNA a key source of aging? Or, alternatively, are changes in the rate of transcription or translation of the information in DNA a key cause? Medvedev (32) was the first to propose that the stability of DNA was responsible for the length of life of different species. Orgel then proposed his “error theory of aging” in which he proposed that errors in DNA replication, transcription of RNA, and translation on the products might be responsible for the deterioration of function during aging (33). Over a number of years a major effort was made in this author’s laboratory to test the idea that development and aging were caused by changes in the specific codons different kinds of cells were able to translate. Initial studies showed that the aminoacylated tRNA’s for a variety of amino acids differed from one kind of cell to another and a theory called the “Codon Restriction Theory of Development and Aging” was published in Journal of Theoretical Biology (34). The theory was then tested against the actual codon usage of about 100 different messenger RNAs and it was indeed found that certain kinds of gene products (e.g., the globin parts of hemoglobins) do in fact have very similar patterns of codon usages and codon dis-usages in messages ranging from birds (chickens) to mice and rats to humans! On these bases, the inability to translate specific codons in specific kinds of tissues may indeed turn out to be important in the control of gene expression (at least in some tissues). 6. Are changes in RNA qualities responsible for aging? Whether the kinds of RNA present in cells is important in controlling differentiation and aging is an issue that has arisen when it was discovered that certain RNA molecules possess catalytic activity, e.g., are able to generate themselves by catalytically transforming their precursors (35). I have recently read evidence that even the transfer of growing polypeptide chains to the amino acid on the a tRNA to the “next” position is catalyzed in the ribosomes by a particular kind of RNA. Whether changes in catalytic RNA populations cause certain disabilities during aging has not yet been tested, to my knowledge.

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7. Do long-lived cells selectively fail in humans? The answer to this question is certainly yes. The main sites in which clear age changes take place are in cells that cannot be replenished without a disruption in their functions in the body. Key cell types are neurons, heart muscle, skeletal muscle, and certain hormone producing cells. The important precursor of both androgens and estrogens, DHAE, declines linearly with age in men and women and may well be a product of cells that are not replenishable. But even more obvious is the postmitotic nature of cells in the nervous system and other nonreplenishing tissues such as skeletal and heart muscle. Thus, damage to the cells making up these organs generally cannot be repaired through replacement because such postmitotic cells cannot be made to divide. In the case of the brain, continual replacement of old cells by new ones might preserve reflex brain function, but most such newly incorporated nerve cells would replace neurons in whose facilitated synapses useful memories had been stored. Thus, paradoxically, higher animals, particularly humans, age because some key kinds of cells they possess have long, but not indefinitely long, lifetimes. (Although it is fairly obvious I would like it to be called “The Strehler Paradox,” so that way I might be remembered for something unless a different version of the perpetual motion machine I proved unworkable actually generated useful energy!) 8. What are the underlying causes of the age-related decline in the immune system? The immune system consists of two major forms: innate and acquired. Innate immunity comprises polymorphonuclear leukocytes, natural killer cells, and mononuclear phagocytes and utilizes the complement cascade as the main soluble protein effector mechanism. This type of immunity recognizes carbohydrate structures that do not exist on eukaryotic cells; thus foreign pathogens can be detected and acted against. Lymphocytes are the major cells involved in the system of acquired immunity, with antibodies being the effector proteins. The T-cell receptor (TCR) and antibodies recognize specific antigenic structures. Deterioration of the immune system with aging (“immunosenescence”) is believed to contribute to morbidity and mortality in man due to the greater incidence of infection, as well as possibly autoimmune phenomena and cancer in the aged. T lymphocytes are the major effector cells in controlling pathogenic infections, but it is precisely these cells that seem to be most susceptible to dysregulated function in association with aging. Decreases in cell-mediated immunity are commonly measured in elderly subjects. By most parameters measured, T-cell function is decreased in elderly compared to young individuals. Moreover, prospective studies over the years have suggested a positive association between good T-cell function in vitro and

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individual longevity. The numbers and/or function of other immune cells are also altered with age: antigen-presenting cells are less capable of presenting antigen in older age; the number of natural killer cells increases in older age, and these cells are functionally active; there is some evidence that granulocyte function may be altered with age; B lymphocyte responses also alter with age, as responses against foreign antigens decline whereas responses against self-antigens increase (36,37). Currently much effort is being directed toward elucidating the processes leading to the phenomenon of immunosenescence. The reader is encouraged to read a special issue of Mechanisms of Ageing and Development that was dedicated to publishing the proceedings of a recent international meeting on immunosenescence (38). One positive aspect of immunosenescence, however, is that the risk of transplant rejection is reduced with age. 9. Are ordinary mutations a major cause of aging? Or, alternatively, is the instability of tandemly repeated DNA sequences a major cause of aging? In 1995 a Special issue of Mutation Research entitled “Somatic Mutations and Ageing: Cause or Effect?” was published, with an overview from this author highlighting the history of this field of science (39). Much of the early results from the experiments on the effects of ionizing radiation and chemical mutagens on the life-span of Drosophila and other animals were inconsistent with a simple mutation theory of aging. However, the research papers presented in the special issue of Mutation Research, and elsewhere, do suggest an involvement of somatic and mitochondrial mutation in the physiological and pathological decline associated with the aging process. I also believe that some other kind of DNA change, the occurrence of which was not accelerated by radiation proportionally to dose (as are ordinary mutations), could be responsible for aging. This kind of postulated change in DNA might well occur sufficiently frequently, even in unirradiated animals, to cause aging! In humans the nucleolar organizing regions (NORs), which can be detected by silver staining, are regions containing rDNA which is the template on which rRNA is formed. There are about five or six pairs of chromosomes that possess such NOR regions. It has been shown that the number of NORs decreases with time in a variety of human cells. Perhaps, I thought, losses of such tandemly duplicated regions takes place at a relatively high rate in nondividing human cells during aging, but is not appreciably increased by exposure to moderate amounts of radiation. After all, radiation affects all kinds of DNA and the rDNA genes may well be able to repair most of the damage they receive either during aging or as a result of chemical or electromagnetic radiation such as UV light and X-rays or by neutrons. I postulated that mutations that cause the loss of rDNA might be responsible for human aging because the more severe such loss

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is, the greater should be the loss of function of any cell in manufacturing proteins. Such mutations could be the kind that cause the linear decrease in function of various parts of the body observed by Shock. Although I thought this unlikely to occur, particularly in postmitotic cells, we were eager to disprove it, because loss of important genetic material would be very difficult to reverse (e.g., through the use of a “clever” virus), whereas a defect in the regulation of gene expression which had been the focus of our research should require simpler, but presently unknown, treatments to modify the rate of aging—which at that time seemed to be on the horizon. To test the possibility that rDNA loss is a major cause of aging, I asked a very talented postdoctoral trainee, the late Roger Johnson, to work to study the rDNA content of various mammalian tissues. He owned a small airplane that made it possible for him to fly to Davis, California to obtain a variety of tissues of control beagle dogs of different ages that were killed as part of an ongoing study by the Atomic Energy Commission to determine the pathological effects of radiation. We obtained fresh samples from the following organs: brain, heart, skeletal muscle, kidney, spleen, and liver. When we compared the rDNA content of the brains of beagles of various ages we found that the results were not what we had hoped for and expected—namely, that no difference would be found between young and old animals. Instead, the findings were that the rDNA content decreased by about 30% in brains of dogs from approx 0–10 yr of age (40). We then proceeded to compare the effect of age on the DNA of heart, skeletal muscle, kidneys, spleen, and liver. Decreases in rDNA of about the same magnitude were found in the other two postmitotic tissues, heart and skeletal muscle, but were not detected in liver DNA or kidney DNA. A small, probably insignificant, loss of these gene sequences was detected in dog spleens (41,42). After the work on dogs was completed, we began to study human heart and found a substantial loss of rDNA of aged humans (43). We later studied two different areas of the human brain, the somatosensory cortex and the hippocampus. The fresh autopsy samples were kindly supplied by the Los Angeles coroner. We discovered that the rate of loss of rDNA from human brain and heart was about 70% per 100 yr. This rate is only about 1/7th of the rate observed in dogs and thus is inversely proportional to the maximum longevity of these two species (approx 120 yr and approx 16 yr). The ratio of these two life-spans is very close to 7:1 and the ratio of loss of rDNA/yr is about 1:7. The two parts of the human brain measured were almost identical in their rDNA content, although the loss was of course greater in old tissues than in young ones. This indicates that the measurements are reliable or at least that, if errors were made, the errors must be very small. Over a period of about 10 yr we continued to publish studies on humans. Most of these studies were reported in Mechanisms of Aging and Development.

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A very interesting class of mutants in Drosophila are called the minute mutants. My former dear friend Kimball Atwood (who has departed to the great genetics lab in the sky) noted that there are many different minute mutants and that they are found in various places on essentially all of the four chromosomes this animal possesses. He suggested that the mutants might reflect the loss of at least part of the tRNA coding regions for specific tRNAs. I don’t know whether this hypothesis has been critically tested—if it hasn’t it certainly should be. Deficiency (but not total absence of specific tDNAs that decode specific amino acids) would be expected to interfere with the normal growth rate of all parts of the developing fly embryo—hence the name, minute. 10. What causes Alzheimer’s disease and cancers—and what means are now available to control these tragic diseases of the elderly (and of certain younger persons as well)? I spent considerable time and effort recently studying another major scientific question: Is a specific temporal code used in transmitting, decoding, and storing information (memories) in the mammalian brain? I had published a theory on this concept in Perspectives in Biology and Medicine in 1969 (44). Knowledge of such a coding system could be quite interesting and probably useful in understanding the familial forms of Alzheimer’s disease. I studied the patterns in time of nerve discharges in response to specific stimuli to the eyes of monkey brains. In the meantime I had constructed an electronic memory system I thought might mimic the brain. I made some progress and wrote a program that serially mimicked how I thought the brain might store and recognize patterns. I also constructed an electronic analogue that worked quite well. But, I made little progress in obtaining a clear answer regarding the validity of my hypothesis until a brilliant French scientist, Dr. Remy Lestienne, wrote to ask whether he could spend a year working with my “group” (at that time only me!). After we had worked together for only a month we discovered that the brain really did produce extremely precise copies of doublets, triplets, quadruplets, and even sextuplets of pulses. Then we analyzed various parameters, including the decay time for the occurrence of repeating patterns. The patterns we used were precisely repeated with variances between copies of the same pattern of less than 1/7th of a millisecond for each of the three intervals that make up a pattern. This was most surprising, because the duration of a nerve impulse is about 1 ms. Perhaps the most important discovery we made was that each repeating triplet was surrounded by about seven doublets that were part of the repeating pattern and equally precisely replicated. Thus, we had not disproved my theory, but rather found evidence that it was probably correct, at least for short-term memories.

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While this research was going on I also developed an electronic simulation of the basic concepts and obtained a U.S. Patent on this device in 1993. I also received a second patent that proposed a means to recognize different vowels on the basis of the differences in logarithms of frequencies generated within the mouth and nasopharyngeal cavities. Because the absolute frequencies that children and women and men use to produce vowels are quite different a puzzle existed as to how different vowels are understood despite the fact that the absolute frequencies generated are much different from person to person. A largescale implementation of the content addressable temporal coding has not been implemented although a very simple version was constructed by me and an improved version was created by a most ingenious Japanese engineer named Yuki Nakayama (sponsored by my friend H. Ochi, who has a consuming interest in aging research and is quite wealthy.) Perhaps the very new CD recorder that Sony has recently marketed may be modified to construct a new and inexpensive way to implement a device able to store the 1014 bits the human brain evidently can store and retrieve upon proper cueing. Alzheimer’s disease is manifested by the loss of memory, initially that involving the recent past. One can remember minuscule details of the more distant past, but sometimes forgets what day of the week it is and what one wanted to get from the kitchen when one gets there. This realization of defects in remembering recent events can be quite disconcerting to those of us who have enjoyed the use of memory, logic, and analogy in solving scientific problems and important problems generated by the process of getting older. Alzheimer’s is also called presenile dementia, which means that it can occur as early as the late 40s or 50s, long before other signs of senility manifest themselves. As the disease progresses victims may even lose the ability to recognize family members or even their spouses or their own names. When the brains of persons who die of various diseases are autopsied, it is possible to recognize those who have advanced stages of Alzheimer’s degeneration by looking for the many “plaques” characteristic of Alzheimer’s. Similar plaques are found in the brains of essentially all very elderly persons, but they are markedly more numerous in the brains of true inheritors of the acute form of this age change in brain anatomy—persons with Alzheimer’s. The plaques are visible on the surface of the brain and consist of localized patches of changed brain tissue visible to the naked eye. When the plaques are examined microscopically at least three characteristics are obvious: (1) the plaques contain many dead or dying cells; (2) most of the cells that are still alive in a plaque possess long tangles of fibers that are not found in profusion in “normal” neurons elsewhere in the same individual’s brain biopsy; and (3) the cells are surrounded by very large accretions of antibody-like substances called amyloid. These deposits often encase the entire cell body of a neuron. It is important to note that these amyloid

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deposits are evidently different from most other kinds of amyloid found in the brain and elsewhere. The key difference appears to be that a cleavage product of the amyloid characteristic of Alzheimer’s causes cell death by opening Ca2+ channels in the neural membranes’ neuroreceptor regions. This causes a permanent depolarization of the cells and evidently is the cause of their death and the loss of memories the cells or cell groups store. The most exciting research on this subject of which I am aware is that the drug Flupirtine now used in Europe for the treatment of Alzheimer’s, reportedly with some success, prevents the influx of Ca2+ into cells that are pretreated with this substance when Alzheimer’s amyloid is presented to them. This work, recently published in Mechanisms of Ageing and Development, was carried out by my good friend, Werner Mueller, who will become Editor-in-Chief of the journal when I cease my editorial responsibilities at the end of this year (45). I believe this is the most significant finding to be published on possible treatment of a very sad disease of the elderly. Cancer is a common cause of morbidity and mortality in the elderly. The spectrum of the major types of cancers occurring in the early years of life (leukemias and sarcomas) is different from that occurring in later life (carcinomas and lymphomas). The most frequent cancers in women in Western societies are breast, ovarian, and colorectal, and in men prostate, lung, and colorectal. The multistep theory of carcinogenesis (46) predicts the age-related increased risk (5th power of age in both short-lived species such as rats and long-lived species such as humans) for the development of a wide range of different types of cancer (with the exception of the familial forms of the disease). The underlying molecular cause of cancer is the accumulation of mutations within a number of genes associated with the control of cell growth, division, and cell death. Despite the great variety of cells that can give rise to cancer there are now somewhat effective treatments for many of them (surgery, radiotherapy, and/or chemotherapy). Optimal treatment for many cancers is more likely the earlier the diagnosis is made. Among the most promising of new treatments for some cancers is the use of radioactively labeled antibodies to the surface antigens present on some cancer cells but not on normal cells. The labeled antibody seeks out the surface of the cancer cell and the radioactivity attached to it selectively radiates and destroys the tumor cells. Another recent treatment that appears to have at least some success is the use of substances that prevent angiogenesis, thereby effectively “asphyxiating” the dangerous tumor. References 1. Weissmann, A. (1891) Essays on Heredity and Kindred Biological Problems, Oxford University Press, Clarendon, London, and New York.

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2. Pearl, R. (1921) The biology of natural death, public policy and the population problem. Sci. Month. 193–212. 3. Carrel, A. (1935) Man the Unknown, Harper, New York. 4. Comfort, A. (1956) The Biology of Senescence, Rinehart, New York. 5. Strehler, B. L. (1962) Time, Cells and Aging, 1st ed., Academic Press, New York. 6. Strehler, B. L. (1977) Time, Cells and Aging, 2nd ed., Academic Press New York. 7. Strehler, B. L. (1999) Time, Cells and Aging, 3rd ed., Master Print Demetriades Bros., Cyprus. 8. Timeras, P. (1972) Decline in homeostatic regulation, in Developmental Physiology and Aging (Timeras, P. S., ed.), MacMillan Press, New York, pp. 542–563. 9. Gompertz, B. (1825) On the nature of the function expressive of the law of human mortality. On a new mode for determining life contingencies. Philos. Trans. R. Soc. Lond. 115, 513–585. 10. Norris, A., Shock, N., and Wagman, I. (1953) Age changes in the maximum conduction velocity of motor fibers of human ulnar nerve. J. Physiol. 5, 589–593. 11. Shock, N. and Yiengst, M. (1955) Age changes in basal respiratory measurement and metabolism in males. J. Gerontol. 10, 31–40. 12. Mildvan, A. S. and Strehler, B. L. (1960) A critique of theories of mortality, in The Biology of Aging (Strehler, B. L., et al., eds.), Publ. No. 6, Am. Inst. Biol. Sci., Washington, D.C., pp. 399–415. 13. Strehler, B. L. (1960) Fluctuating energy demands as determinants of the death pricess (a parsimonious theory of the Gompertz function in (Strehler, B. L., et al., eds.), Publ. No. 6, Am. Inst. Biol. Sci., pp. 309–314. 14. Strehler, B. L. and Mildvan, A. (1960) A general theory of mortality and aging. Science 132, 14–21. 15. Strehler, B. L. (1961) Studies on the comparative physiology of aging. II. On the mechanism of temperature life-shortening in Drosophila melanogaster. J. Gerontol. 16, 2–12. 16. Strehler, B. L. (1962) Further studies on the thermally induced aging of Drosophila melanogaster. J. Gerontol. 17, 347–352. 17. Loeb, J. and Northrop, J. H. (1916) Is there a temperature coefficient for the duration of life? Proc. Natl. Acad. Sci. USA 2, 456. 18. Loeb, J. and Northrop, J. H. (1917) On the influence of food and temperature on the duration of life. J. Biol. Chem. 32, 103–121. 19. Sinex, F. M. (1964) Cross linkage and aging. Adv. Gerontol. Res. 1, 167–178. 20. Houuck, G. Dehesse, C., and Jacob, R. (1967) The effect of aging upon collagen catabolism. Symp. Soc. Exp. Biol. 21, 403–426. 21. Franzblau, C. and Lent, R. (1968) Studies on the chemistry of elastin. Brookhaven Symp. Biol. 21, 358–377. 22. Gallop, P., Blumenfeld, O., Henson, E., and Schneider, A. (1968) Isolation and identification of alpha amino aldehyde and collagen. Biochemistry 7, 2409–2430. 23. Franzblau, C. Fariz, P., and Papaioannou, R. (1969) Lysenonorleucine. A new amino acid from hydrolysate of elastin. Biochemistry 8, 2833–2837.

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24. McElroy, W. (1947) The energy stored for bioluminescence in an isolated system. Proc. Natl. Acad. Sci. USA 33, 342–345. 25. McElroy, W. and Strehler, B. L. (1949) Factors influencing the response of the bioluminescent reaction to ATP. Arch. Biochem. 22, 420–433. 26. Strehler, B. L. and Totter, J. R. (1962) Firefly luminescence in the study of energy transfer mechanisms. 1. Substrate and enzyme determination. Arch. Biochem. Biophys. 40, 28–41. 27. Fleming, J. E., Miquel, J., Cottrell, S. F., Yenguyan, L. S., and Economas, A. S. (1982) Is cell aging caused by respiration dependent injury to the mitochondrial genome? Gerontology 28, 44–53. 28. Hayflick, L. (1965) The limited in vitro lifetime of human diploid strains. Exp. Cell Res. 37, 614–638. 29. Hay, R. L., Menzies, R. A., Morgan, H. P., and Strehler, B. L. (1967) The division potential of cells in continuous growth as compared to cells subcultured after maintenance in stationary phase. Exp. Gerontol. 35, 44. 30. Chang, E. and Harley, C. B. (1995) Telomere length and replicative aging in human vascular tissues. Proc. Natl. Acad. Sci. USA 92, 11,190–11,194. 31. Bodnar, A. G., Ouellette, M., and Frolnis, M. (1998) Extension of lifespan by introduction of telomerase into normal human cells. Science 279, 349–352. 32. Medvedev, Sh. A. (1961) Aktual. Vopr. Sour. Biol. 51, 299. 33. Orgel, L. E. (1970) The maintenance of the accuracy of protein synthesis and its relevance to aging: a correction. Proc. Natl. Acad. Sci. USA 67, 1426–1429. 34. Strehler, B. L., Hirsch, G., Gusseck, D., Johnson, R., and Bick, M. (1971) The codon restriction theory of aging and development. J. Theor. Biol. 33, 429–474. 35. Andron, L. and Strehler, B. L. (1973) Recent evidence on tRNA and tRNA acylasemediated cellular control mechanisms. A review. Mech. Ageing Dev. 2, 97–116. 36. Pawelec, G. and Solana, R. (1997) Immunosenescence. Immunol. Today 18, 514–516. 37. Pawelec, G., Remarque, E., Barnett, Y., and Solana, R. (1998) T cells and aging. FIBS 3, 59–99. 38. Special edition of Mechanisms of Aging and Development (1998), 102. 39. Special issue of Mutation Research (1995) Somatic Mutations and Aging: Cause or Effect? 338, 1–234. 40. Johnson, R. and Strehler, B. L. (1972) Loss of genes coding for ribosomal RNA in aging brain cells. Nature (Lond.) 240, 412–414. 41. Johnson, R., Chrisp, C., and Strehler, B. L. (1972) Selective loss of ribosomal RNA genes during the aging of post-mitotic tissues. Mech. Ageing Dev. 1, 183–198. 42. Johnson, L. K., Johnson, R. W., and Strehler, B. L. (1975) Cardiac hypertrophy, aging and changes in cardiac ribosomal RNA gene dosage in man. J. Mol. Cell. Cardiol. 7, 125–133. 43. Strehler, B. L., Johnson, L. K., and Chang, M. P. (1979) Loss of hybridizable ribosomal DNA from human postmitotic tissues during aging: I Age-dependent loss in human myocardium. Mech. Age. Dev. 11, 371–378. 44. Strehler, B. L. (1969) Information handling in the nervous system: an analogy to molecular genetic coder-decoder mechanisms. Perspect. Biol. Med. 12, 548–612.

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45. Perovic, S., Böhm, M., Meesters, E., Meinhardt, A., Pergande, G., and Müller, W. F. G. (1998) Pharmacological intervention in age-associated brain disorders by Flupirtine: Alzheimer’s and Prion diseases. Mech. Age. Dev. 101, 1–19. 46. Vogelstein, B. and Kinzler, N. W. (1993) The multistep nature of cancer. TIG 9, 138–141.

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2 Use of the Fibroblast Model in the Study of Cellular Senescence Vincent J. Cristofalo, Craig Volker, and Robert G. Allen 1. Introduction In this chapter, we present standard procedures for the culture of human cells that exhibit a finite proliferative capacity (replicative life-span). The use of a cell culture model has the advantage of providing a controlled environment to study a wide variety of cellular phenomena. It also has the inherent limitation of isolating cells from the regulatory elements that might be provided by other types of cells in vivo. Nevertheless, cell culture models have been crucial to our current understanding of mechanisms of growth, differentiation, development, and neoplasia and numerous other disease states. In this chapter we present procedures for human fibroblast culture including serumfree cultivation of cells, which is necessary when the cellular environment must be fully defined. In addition, we present procedures for the determination of replicative life-span, saturation density, and assessment of replicative capacity from labeled thymidine incorporation in fibroblasts. The methods described here have been well tested and provide highly reproducible results (1,2).

1.1. Cellular Senescence Phenotypically and karyotypically normal human cells exhibit a limited capacity to proliferate in culture (3,4). This finite proliferative potential of normal cells in culture is thought to result from multiple changes (5) and has frequently been used as one model of human aging. Although most replicative life-span data are derived from fibroblasts, other types of cells such as glial cells (6), keratinocytes (7), vascular smooth muscle cells (8), lens cells (9), endothelial cells (10), lymphocytes (11), liver (12), and melanocytes (13) are also known to exhibit a limited replicative life-span in culture. Both environFrom: Methods in Molecular Medicine, Vol. 38: Aging Methods and Protocols Edited by: Y. A. Barnett and C. R. Barnett © Humana Press Inc., Totowa, NJ

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mental and genetic factors appear to influence the proliferative life-span of fibroblasts from normal individuals (5,14,15). Not all of the determinants of proliferative capacity are known; however, a variety of changes are associated with the decline of proliferative capacity including changes in gene expression, telomere shortening, and signal transduction. These are all thought to be important factors that influence replicative life-span (15–20).

1.1.1. Telomere-Shortening Loss of telomeric repeats is tightly linked to the cessation of mitotic activity associated with cellular senescence (16,17,21,22). The telomeres of human chromosomes are composed of several kilobases of simple repeats (TTAGGG)n. Telomeres protect chromosomes from degradation, rearrangements, end-to-end fusions, and chromosome loss (23). During replication DNA polymerases synthesize DNA in a 5' to 3' direction; they also require an RNA primer for initiation. The terminal RNA primer required for DNA replication cannot be replaced with DNA, which results in a loss of telomeric sequences with each mitotic cycle (21,23). Cells expressing T antigen are postulated to exhibit an increase in their proliferative life-span because they are able to continue proliferating beyond the usual limit imposed by telomere length (24). Immortalized and transformed cells exhibit telomerase activity that compensates for telomere loss by adding repetitive units to the telomeres of chromosomes after mitosis (23,25–27). Cultures derived from individuals with Hutchinson–Gilford syndrome (28) often exhibit decreased proliferative potential, albeit results with these cell lines are variable (29). Fibroblast cultures established from individuals with Hutchinson–Gilford progeria syndrome that exhibit a lower proliferative capacity than cells from normal individuals also exhibit shorter telomeres; however, the rate of telomere shortening per cell division appears to be similar in progeria fibroblasts and normal cells (16). It has recently been demonstrated that proliferative senescence can be delayed and possibly eliminated by transfection of normal cells with telomerase to prevent telomere loss (30). It is also interesting to note that other repetitive DNA sequences become shorter during proliferative senescence (31,32)

1.1.2. Mitogenic Responses and Signal Transduction As a result of senescence-associated changes, cells assume a flattened morphology and ultimately cease to proliferate in the presence of serum (5). Numerous factors may contribute to the senescent phenotype; however, the principal characteristic of cellular senescence in culture is the inability of the cells to replicate DNA. Paradoxically, the machinery for DNA replication appears to remain intact, as indicated by the fact that infection with SV-40 initiates a round of semiconservative DNA replication in senescent cells (33).

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Nevertheless, senescent cells fail to express the proliferating cell nuclear antigen (PCNA), a cofactor of DNA polymerase δ, apparently as a result of a posttranscriptional block (34). Furthermore, senescent fibroblasts fail to complement a temperature-sensitive DNA polymerase α mutant (35,36). This may contribute to the failure of senescent cells to progress through the cell cycle because it is known that a direct relationship exists between the concentration of DNA polymerase α and the rate of entry into S phase (37). It has also been observed that replication-dependent histones are also repressed in senescent cells and that a variant histone is uniquely expressed (18). It might also be noted that the senescence-dependent cessation of growth is not identical to G0 growth arrest that occurs in early passage cells that exhibit contact inhibited growth or that are serum starved. Several lines of evidence suggest that senescent cells are blocked in a phase of the cell cycle with many characteristics of late G1. For example, thymidine kinase is cell cycle regulated; it appears at the G1/S boundary. Thymidine kinase activity is similar in cultures of proliferating young and senescent WI-38 cells (38,39). It should also be noted that thymidine triphosphate synthesis, which normally occurs in late G1, is not impaired in senescent cells (39). Furthermore, the nuclear fluorescence pattern of senescent cells stained with quinacrine dihydrochloride is also typical of cells blocked in late G1 or at the G1/S boundary (33,40). In addition, Rittling et al. (41) demonstrated that 11 genes expressed between early G1 and the G1/S boundary are mitogen inducible in both young and senescent cells. On the other hand, growth-regulated genes such as cdc2, cycA, and cycB, which are expressed in G1, are repressed in senescent cells (42). These observations suggest the possibility that senescent cells are irreversibly arrested in a unique state different from the normal cell cycle stages. As cells approach the end of their proliferative potential in culture they become increasingly refractory to mitogenic signals (15,43,44). The signal transduction pathways that convey these mitogenic signals play significant roles in the regulation of cell proliferation and adaptive responses; hence, decline in the activity of elements in these pathways may contribute significantly to the senescent phenotype. For example, there is a senescence-associated loss in the capacity of cells to activate protein kinase C (45) or to increase interleukin-6 (IL-6) mRNA abundance (46) following stimulation with phorbol esters. Furthermore, transcriptional activation of c-fos following stimulation of cultures with serum is also diminished in senescent cells (18,47). Other genes such as Id1 and Id2, which encode negative regulators of basic helix–loop– helix transcription factors, fail to respond to mitogens in senescent cells (48) Although signal transduction efficiency declines with replicative age, the members of affected pathways are seldom influenced uniformly by senescence. For example, both the number of receptors (per unit cell surface area) and

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receptor affinities for epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and insulin-like growth factor-one (IGF-one) remain constant throughout the proliferative life of fetal lung WI-38 fibroblasts (49–51); however, senescent WI-38 cells produce neither the mRNA nor the protein for IGF-I (52). Similarly, young and senescent WI-38 fibroblasts have similar baseline levels of intracellular Ca2+ and exhibit similar changes in cytosolic Ca2+ fluxes following growth factor stimulation (53); however, the expression of calmodulin protein is uncoupled from the cell cycle and exists in variable amounts in senescent WI-38 cells (53). The calmodulin-associated phosphodiesterase activity also appears to be diminished in late-passage cells (Cristofalo et al., unpublished results). At least some of the changes in signal transduction associated with senescence may also stem from alterations in the cellular redox environment, because the rate of oxidant generation increases during senescence (54) and some steps in various signal transduction pathways are highly sensitive to changes in redox balance. The protein abundances of protein kinase A (PKA) and various isoforms of protein kinase C (PKC) are unchanged or slightly increased by senescence (20,55); however, PKC translocation from the cytoplasm to the plasma membrane is impaired in senescent fibroblasts (45,56). Changes in signal transduction efficiency associated with senescence are not necessarily the result of any decrease or loss of components of signaling pathways. Experiments performed in various types or immortal and normal cells reveal that increases in signal transduction components can also impede signaling pathways. This is most clearly seen in the case of the extracellular signal-regulated kinase (ERK) pathway where the correct sequence and duration of activation and inactivation of ERKs at the G1/S boundary (57–59) is required for entry into S phase. Indeed, constitutive ERK activation has an inhibitory effect on cell cycle progression, both in NIH 3T3 fibroblasts (58) and in Xenopus oocytes (60). Furthermore, overexpression of oncogenic ras in human fibroblasts leads to a senescent-like state rather than to an immortal phenotype (61). Thus, increases as well as decreases in individual components of pathways may contribute to senescence-associated changes in signal transduction. Taken together, senescence-associated changes in mitogenic signaling pathways occur for a variety of reasons that may include any imbalances in or dysregulation of controlling pathways. Interestingly, these effects are largely confined to proliferation and noncritical functions because, if maintained, subpopulations of cells can survive indefinitely in a senescent state.

1.2. Relevance to Aging Before beginning our discussion of methods for the propagation of human fibroblasts and determination of replicative life-span, we digress briefly to discuss interpretation of this type of data. We shall also consider the relationship

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between changes observed during senescence in vitro and aging in vivo. Finally, we will examine a second hypothesis that suggests that senescence in vitro recapitulates at least some aspects of developmental changes associated with differentiation. The finite replicative life-span for normal cells in culture is thought to result from multiple environmental and genetic mechanisms (5) and has frequently been used as a model of human aging. Historically the use of replicative lifespan of cell cultures as a model for aging has been accepted because (1) fibroblast replicative life-span in vitro has been reported to correlate directly with species maximum life-span potential (62), and most importantly (2) cultures of normal human cells have been reported to exhibit a negative correlation between proliferative life-span and the age of the donor from whom the culture was established (8,16,63–68). Other types of evidence also appear to support the strength of the model. For example, the colony-forming capacity of individual cells has also been reported to decline as a function of donor age (69,70). Various disease states of cell donors have been found to significantly influence the proliferative life-spans of cells in culture. For example, cell strains established from diabetic (68,71) and Werner’s patients exhibit diminished proliferative potential (19,28,65,72,73). Cultures derived from individuals with Hutchinson–Gilford syndrome (28) and Down’s syndrome (28,74) may also exhibit decreased proliferative potential, albeit results with these cell lines are more variable (29). Collectively, these observations have been interpreted to suggest that the proliferative life-span of cells in culture reflects the physiological age as well as any pathological state of the donor from which the cells were originally obtained. It must be noted that interpretation of replicative life-span data is often difficult owing to large individual variations and relatively low correlations. For example, one large study (75) determined replicative life-span in more than 100 cell lines, yet obtained a correlation coefficient of only–0.33. Hence, it is difficult to assess whether the reported negative correlations between donor age and replicative life-span indicate any compromise of physiology or proliferative homeostasis in vivo (75,76). A major factor that has influenced the results of most studies is the health status of donors when tissue biopsies were taken to establish the cell cultures (68,75). Most studies include cell lines established from donors who were not screened thoroughly for disease, as well as cell lines derived from cadavers to determine the effects of donor age on proliferative potential. Variations in the biopsy site have also been a factor that probably influenced the results of many studies (68,75). Studies of rodent skin fibroblasts appear to support the existence of a small, but significant, inverse correlation between donor age and replicative life-span (67,77,78). Furthermore, it has also been observed that treatment of hamster

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skin fibroblasts with growth promoters can extend the proliferative life of cultures established from young donors but has negligible effects on cultures established from older donors (79). Aside from inherent species differences and the effects of inbreeding that may influence these results, it is also apparent that rodent skin is better protected from some types of environmental injury such as light exposure. However, even in rodents, the relationship between donor age and proliferative potential is not entirely clear. For example, an examination of hamster skin fibroblast cultures established from the same donors at different ages reveals no age-associated changes in proliferative potential in animals older than 12 mo (78). To address these issues, we recently examined the proliferative potential of 124 human fibroblast cell lines from the Baltimore Longitudinal Study of Aging (BLSA) (80). All of these cell lines were established from donors described as healthy, at the time the biopsy was taken, using the criteria of the BLSA. This study revealed no significant change in proliferative potential of cell lines with donor age, nor did we observe a significant difference between fetal and postnatally derived cultures (80). Goldstein et al. (68) also reported that no relationship between proliferative life-span and donor age could be found in healthy donors but did observe a relationship in diabetic donors. In addition, we performed a longitudinal study by determining the replicative life-span of cell lines established from individuals sampled sequentially at different ages. As in the case of the cross-sectional analysis, no relationship between donor age and replicative potential was found in this longitudinal study. Indeed, cell lines established from individuals at older ages frequently exhibited a slightly greater proliferative potential than the cell lines established from the same individuals at younger ages (80).

1.2.1. Relationship of In Vitro and In Vivo Models One of the underlying assumptions of in vitro aging models is that the changes observed during proliferative senescence bear at least some homology to those observed during aging in vivo. In fact, both similar (concordant) and dissimilar (discordant) changes have been observed between aging-associated changes observed in vivo and in vitro; these are summarized in Table 1. Although the results presented in Table 1 clearly demonstrate that some similarities do exist between aging in vivo and replicative senescence, it remains unclear whether these changes arise through a common mechanism or via distinct pathways. As seen in Table 1, senescence in tissue culture and aging in the intact organism are not homologous. Others have noted that progressive morphological changes begin to develop in diploid cell cultures shortly after they are established regardless of the donor age; no cells are found in vivo

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Table 1 Aging in Cell Culture Replicative Senescence vs Donor Age Concordant features

Discordant features

Collagenase Stromelysin PAI-1

(↑)a (81,82) (↑) (85) (↑) (88,89)

c-fos induction EPC-1 mRNA H-twist mRNA

(↓) (↓) (↓)

IGF-BP3 TIMP-1 HSP 70 Response to Ca2+

(↓) (↓) (↓) (↓)

G-6-PDH mRNA Fibronectin ND-4 mRNA p21 mRNA MnSOD mRNA β-Galactosidase

(=)b (↑) (↑) (↑) (↑?) (↑)

Chemiluminescence H2O2 Generation Collagen a(1)I mRNA Proliferative capacity Saturation density

(↑) (↑) (↓) (↓) (↓)

(91) (85) (94,95) (98,99)

(20,83,84) (86,87) (90; unpublished) (54,92) (93) (96,97) (100,101) (102,103) (Cristofalo, unpublished) (54,96) (54,96) (100,104,105) (80) (80)

aArrow

indicates direction of change in replicative senescence. no change. G-6-PDH, glucose-6-phosphate dehydrogenase; HSP 70, heat shock protein 70; IGF-BP3, insulin-like growth factor binding protein-3; PAI-1, plasmogen activator inhibitor-1; SOD, superoxide dismutase; TIMP-1, tissue inhibitor of metalloproteinase-1. bIndicates

at any age that exhibit the morphological phenotype of cells, in vitro, at the end of their replicative life-span (106). Rubin (76) suggests that the limited replicative life-span in vitro may be an artifact that reflects the failure of diploid cells to adapt to the trauma of dissociation and the radically foreign environment of cell culture. However, that hypothesis ignores factors such as telomere shortening that appear to influence proliferative life and that are not dependent on the culture environment. Presently, it is possible to state that the loss of proliferative potential in vitro does not directly reflect changes in replicative capacity that occur in vivo during aging and that changes in gene expression associated with replicative senescence are not completely homologous with changes observed during aging in vivo.

1.2.2. Relationship Between Senescence and Development One view of the limited proliferative capacity of cells in culture is that it stems from the effects of the culture environment on the state of differentiation of the cells (107–113). Although the state of differentiation may change in

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Table 2 Comparison of Replicative Senescence of Fetal Cells In Vitro with Differences Between Fetal and Adult Cells? Concordant ND-4 mRNA MnSOD activity Catalase activity IL-1α IL-1β Response to PDGF-BB

Discordant (↑) (↑) (↑) (↑) (↑) (↑)

(96,97) (103,114) (92) (103,115) (103,115) (116)

c-fos induction EPC-1 mRNA Cu/Zn SOD mRNA MnSOD mRNA Cu/Zn SOD activity COX-1 mRNA

(=)b (↑) (↑) (↑) (↑) (↑)

(84) (86,87,97) (102)c (102,114)c (102)c (96)c

SDd mRNA COX activity NDd activity SD activity G-6-PDH mRNA PDGF requirement Collagen a(1)I mRNA β-Actin

(↑) (↑) (↑) (↑) (↑) (↑) (Ø) (Ø)

(96)c (96)c (96)c (96)c (92) (117) (100,118) (100; unpublished)

aArrow

indicates direction of difference between proliferatively young fetal and adult cells. no change. cBased on observations of changes during proliferative senescence, made in this laboratory that will be presented elsewhere (54). dND=NADH dehydrogenase; SD=succinate dehydrogenase. bIndicates

cells that senesce in vitro, there is, in fact, no evidence that the changes in gene expression observed in fetal cells as they senesce in vitro, are tantamount to differentiation, in vivo. While some analogous changes can be found they are greatly outnumbered by the discordant differences that characterize these two distinct phenomena. Hence, a comparison of senescence-associated changes and differences that exist between fetal and postnatal cells reveals little similarity (Table 2). At least some analogous similarities exist between senescence in fetal fibroblasts and developmental changes that occur in vivo. For example, it has been observed that addition of PDGF-BB stimulated an increased mRNA abundance of the transcript encoding the PDGF-A chain in fetal and newborns; however, the response was greatly decreased in adult cells. Senescence in vitro of newborn fibroblasts appears to result in the acquisition of the adult phenotype (116). In contrast, there are a number of differences reported between fetal- and adultderived cell lines related to growth factor requirements for proliferation and migration (117,119–121) that remain disparate even as these cultures become

Fibroblast Model for Cell Senescence Studies

31

senescent. For example, Wharton (119) has shown that fetal dermal fibroblasts will proliferate in plasma or serum while adult dermal fibroblasts require serum. It is also noteworthy that the expression of some genes, such as SOD-2, increases during proliferative senescence but only in some types of fibroblasts (114); in other types of fibroblasts no change is observed (54,114). It might be expected that cells placed in culture will be deprived of those signals that direct the normal sequence of developmental pathways and that differentiation, if it occurs, is to an aberrant state. Alternatively, fetal cell lines may arise from different precursor cells than do adult fibroblasts and thus merely differentiate to a different fibroblast type.

1.2.3. Limitations and Strengths of the System Although the loss of proliferative potential in vitro may not directly reflect changes in replicative capacity that occur in vivo during aging, cell cultures remain a powerful tool for a variety of aging-related studies. These include studies of heritable damage to cell populations that simulate the effects of aging in vivo (76), a variety of chemical and molecular manipulations used to induce a senescence phenotype, the effects of stress (61,76,122–125), and as a system to study abnormal growth or quiescence (5). The model may also help to further elucidate the effects of diseases that alter proliferative life-span (19,28,65,68,71–73,126). Loss of capacity for senescence is a necessary step for immortalization and transformation to a malignant phenotype. The model may also prove useful in studies of the relationship between differentiation and replicative aging (117,119–121). 2. Materials The serum-supplemented and serum-free, growth factor-supplemented formulations presented each give optimal growth of human diploid fibroblast-like cells. We also present methods for growth of cells in a defined medium using a defined growth factor cocktail (2,127). All reagents and materials for cell culture must be sterile, and all manipulations should be performed in a laminar flow hood. Serial propagation is generally performed in serum-supplemented media, yet serum is a complex fluid with numerous known and unknown bioactive components. For many studies, it is often desirable if not crucial to use a serum-free growth medium of defined composition.

2.1. Serum-Supplemented Medium Suppliers and more detailed information on the items required for the preparation of serum-supplemented media are listed in Table 3. 1. Incomplete Eagle’s modified minimum essential medium: Cells are grown in Eagle’s modified minimum essential medium (MEM). Although the medium can

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Cristofalo, Volker, and Allen

Table 3 Components of Standard Growth Medium Component Auto-Pow™, autoclavable powder Eagle MEM with Earle’s salts without glutamine and without sodium bicarbonate 100× Basal medium Eagle vitamins 200 mM L-Glutamine Sodium bicarbonate (7.5% solution) FBSa aFBS

2.

3.

4.

5.

Amount/L

Supplier

1 pkg 10 mL 10 mL 26 mL 100 mL

ICN ICN Sigma Sigma Various

Cat. no.

11-100-22 16-004-49 G3126 S5761

is from a variety of suppliers and tested on a lot-by-lot basis. See Note 1.

be purchased in liquid form, it is considerably less expensive to prepare the medium from a commercially available mix. In our laboratory incomplete MEM is prepared by dissolving Auto-Pow™ powder (9.4 g) and 100× basal medium Eagle vitamins (10 mL) in 854 mL of deionized, distilled water. After the incomplete medium has been mixed and dissolved, it should be divided into two equal portions (432 mL each) and placed in 1-L bottles (see Note 2). The caps are screwed on loosely, autoclave tape is applied, and the bottles are autoclaved for 15 min at 121°C (see Note 3). As soon as the sterilization cycle is finished, the pressure is quickly released and the bottles are quickly removed from the autoclave. The bottles are allowed to cool to room temperature in a laminar flow hood. When the bottles have cooled, the caps are tightened. Incomplete medium is stored at 4°C in the dark. 100× Basal medium Eagle vitamins: Filter-sterilized 100× basal medium Eagle vitamins are purchased in 100-mL bottles and stored at –20°C. When first thawed, using sterile procedures, the vitamin solution is divided into 10-mL portions and stored in sterile 15-mL centrifuge tubes at –20°C until use. L-Glutamine (200 mM): L-Glutamine (14.6 g) is dissolved in 500 mL of deionized, distilled water without heating. This solution is then sterilized in a laminar flow hood using a 0.2 µm pore size bottletop filter. Aliquots (50 mL) are added to sterile 100-mL bottles that are then capped and stored at –20°C until use. When thawed for use, the glutamine solution is divided into 5-mL portions and stored at –20°C in sterile 15-mL centrifuge tubes until use. Sodium bicarbonate (7.5% w/v): Sodium bicarbonate (37.5 g) is dissolved in 500 mL of deionized, distilled water. This solution is then filter sterilized using a 0.2-µm pore size bottletop filter. The sterile solution is stored at 4°C. Fetal bovine serum (FBS): Prior to purchase, various lots of fetal bovine serum (FBS) are tested for 3 consecutive weeks to determine their effects on the rate of cell proliferation and saturation density. The serum lot that gives the best growth response is chosen, and quantities that will last about 1 yr are purchased. The serum is stored at –20°C until use. Once thawed, serum is stored at 4°C for subsequent use; it should not be refrozen.

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Table 4 Components of Serum-Free Growth Medium Component

Amount

Supplier

Cat. no.

MCDB-104, a modified basal medium with L-glutamine, without CaCl2, without Na2HPO4, without NaHCO3, and without HEPES, and with sodium pantothenate substituted for calcium pantothenate Sodium phosphate, dibasic Sodium chloride Calcium chloride dihydrate Sodium bicarbonate HEPESa 1 M Sodium hydroxidea EGF), human recombinant IGF-I, human recombinant Insulin Ferrous sulfate heptahydrate 1 M Hydrochloric acid Dexamethasone 95% Ethanol (not denatured)

1 pkg/L 0.426 g/L 1.754 g/L 1 mM 1.176 g/L 11.9 g/L 25 mL/L 25 ng/mL 100 ng/mL 5 µg/mL 5 µM Trace 55 ng/mL Trace

Gibco-BRL Sigma Sigma Sigma Sigma Sigma Sigma Gibco-BRL Gibco-BRL Sigma Sigma Sigma Sigma Pharmco

82-5006EA S5136 S5886 C7902 S5761 H9136 S2770 13247-010 13245-014 I6634 F8633 H9892 D4902 111000190CSGL

aNot

used in growth medium. See Note 1.

6. Standard serum-supplemented growth medium (complete medium with 10% v/v FBS): To prepare the standard serum-supplemented growth medium (complete medium with 10% v/v FBS), add 13 mL of filter-sterilized 7.5% (w/v) sodium bicarbonate to 432 mL of sterile, incomplete Eagle’s MEM. The sodium bicarbonate must be added first because low pH can affect glutamine and serum components. After addition of the sodium bicarbonate add 50 mL of sterile FBS. Just before use the medium is prewarmed to 37°C in a warm water bath, then transferred to a laminar flow hood where 5 mL of a 200 mM solution of filter-sterilized L-glutamine is added. Complete medium is generally prepared fresh for each use. If this medium must be stored for periods exceeding 1 wk, additional filter-sterilized L-glutamine (1 mL/100 mL of complete medium) is added just before use.

2.2. Serum-Free Medium Suppliers and more detailed information on the items required for the preparation of serum-free media are listed in Table 4. 1. Serum-free growth medium: This medium is prepared by dissolving a packet of powdered MCDB-104 medium (with L -glutamine, without CaCl2 , without

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Na2HPO4, without NaHCO3, and without N-[2-hydroxyethyl]piperazine-N'-[2ethanesulfonic acid] (HEPES), with sodium pantothenate substituted for calcium pantothenate) in 700 mL of deionized, distilled water. The packet is also rinsed several times to dissolve any medium powder that may have adhered to it. The following additional components are then added in the order listed: 0.426 g of Na2HPO4, 1.754 g of NaCl, 1.0 mL of a 1 M CaCl2 solution, and 1.176 g of NaHCO3. For most studies HEPES is not used. The final volume is brought to 1 L with deionized, distilled water. Incomplete medium is sterilized by filtration through a 0.2-µm bottletop filter into sterile glass bottles. Using sterile procedures in a laminar flow hood, a 5% CO2/95% air mixture is passed through a sterile, cotton-filled CaCl2 drying tube, through a sterile pipet, and bubbled into the medium (see Note 4). As the medium becomes saturated with the gas mixture, its color changes from pink to a salmon color. The final pH is 7.3–7.5. Incomplete medium is generally prepared fresh for each use, but it may be stored for up to 3 wk at 4°C. If unused complete medium is stored longer than 1 wk, additional L-glutamine (1 mL/100 mL of complete medium) should be added before use. 2. HEPES-buffered incomplete medium for stock solutions: The pH of carbon dioxide/bicarbonate-buffered MCDB-104 solutions rises during thawing, resulting in Ca2PO4 precipitate formation. Thus, growth factor and soybean trypsin inhibitor solutions that are stored frozen are prepared in HEPES-buffered solutions. To prepare 1 L of HEPES-buffered incomplete medium, mix medium as described previously except 11.9 g of HEPES free acid and 25.0 mL of 1 M NaOH are added instead of sodium bicarbonate. The pH of the medium is adjusted to 7.5 by titration with additional 1 M NaOH and the volume is brought to a final volume of 1 L with deionized, distilled water. The medium is sterilized by filtration through a 0.2-µm bottletop filter into sterile glass bottles. The HEPES-buffered incomplete medium may be stored at –20°C until needed. 3. Concentrated growth factor stock solutions: For these procedures, use sterile plastic pipets and perform all manipulations in a laminar flow hood. Stock solutions of growth factors (100×) are prepared in HEPES-buffered incomplete medium at the following concentrations: EGF (2.5 µg/mL) and either IGF-I (10 µg/mL) or insulin (500 µg/mL) (see Note 5). All stock solutions are dispensed with sterile plastic pipets into sterile 1.0-mL cryogenic vials. The stock solutions may be stored at –20°C for short periods (up to 4 wk) or at –70°C for longer periods (3–4 mo). Dexamethasone (5 mg/mL) is prepared in 95% nondenatured ethanol. This solution is then diluted into HEPES-buffered incomplete medium to give a 100× stock solution (5.5 µg/mL). Stock dexamethasone is stored in sterile, siliconized test tubes. Ferrous sulfate is prepared fresh, just prior use. After preparation 5 µL of 1 M hydrochloric acid is added to each 10 mL of the ferrous sulfate 100× stock (0.5 mM). This solution is sterilized by filtration through a 0.2-µm filter. 4. Complete serum-free growth medium: For 100 mL of complete serum-free growth medium, 1 mL of each of the 100× stock solutions are added to 96 mL of

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35

Table 5 Components of Trypsinization Solution Component Sodium chloride Potassium chloride Sodium phosphate monohydrate, monobasic 0.14g/L Glucose 50× MEM amino acids, without glutamine 100× basal medium Eagle vitamins 0.5% Phenol red Sodium bicarbonate (7.5% solution) Trypsin, 2.5% in Hanks’ balanced salt solution Soybean trypsin inhibitor, type I-S

Final amount

Supplier

Cat. no.

6.8 g/L 0.4g/L

Sigma Sigma

S5886 P5405

Sigma 1 g/L

S5655 Sigma

G6152

20 mL/L 10 mL/L 10 mL/L 5 mL/50 mL

Gibco-BRL ICN ICN Sigma

11130-051 16-004-49 16-900-49 S5761

5 mL/50 mL 1 mg/mL

Sigma Sigma

T4674 T6522

incomplete medium (MCDB-104). The resultant concentrations in the serumfree medium are: 25 ng/mL of EGF, 100 ng/mL of IGF-I, or 5 µg/mL of insulin (see Note 5); 55 ng/mL of dexamethasone; and 5 µM of ferrous sulfate. 5. Soybean trypsin inhibitor solution for serum-free propagation: Soybean trypsin inhibitor (100 mg) is added to 100 mL of HEPES-buffered incomplete medium. This solution is sterilized by filtration through a 0.2-µm bottletop filter into a sterile bottle. The sterile solution is then dispensed into sterile 15-mL centrifuge tubes in 7-mL portions and stored at –20°C. When needed, the solution is thawed and diluted 1:1 with bicarbonate-buffered incomplete medium.

2.3. Trypsinization Suppliers and more detailed information on the items required for the preparation of trypsinization solution are listed in Table 5. 1. Ca2+/Mg2+-free medium: Cells tend to aggregate in media containing calcium; it is thus desirable to use a medium that is low in Ca2+ and Mg2+ for mixing trypsin solution. To prepare Ca2+/Mg2+-free medium, the following ingredients are added to 900 mL of deionized, distilled water with magnetic stirring: 6.8 g of NaCl, 0.4 g of KCl, 0.14 g of NaH2PO4 · H2O, 1 g of glucose, 20 mL of 50× MEM amino acids without glutamine, 10 mL of 100× basal medium Eagle vitamins, and 10 mL of a 0.5% (w/v) solution of phenol red. The solution is then diluted to 1 L with deionized, distilled water and sterilized by filtration. The Ca2+/Mg2+free medium is stored at 4°C until use. 2. Trypsin stock solution (2.5%): Filter-sterilized trypsin (2.5%) in Hanks’ buffered salts solution is purchased in 100-mL bottles and stored at –20°C. Repeated

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Table 6 Items for Thymidine Incorporation Item

Supplier

Cat. no.

[3H-methyl]-Thymidine, 2 Ci/mmol; 1 mCi/mL Coverslip, No. 1, 22 mm × 22 mm Coverslip rack, ceramic Coverslip rack, glass Chloroform 95% Ethanol, not denatured 95% Sulfuric acid 70% Nitric acid Sodium hydroxide Petri dish, glass, 100 mm NTB-2 Emulsion D-19 Developer Acid fixer Hematoxylin, Harris Modified Permount Microscope slide, 3 in × 1 in Lab-Tek® Chamberslide™, two-chamber Lab-Tek® Chamberslide™, four-chamber Lab-Tek® Chamberslide™, eight-chamber Sodium phosphate, dibasic Potassium phosphate, monobasic Methanol Slide mailer, polypropylene Slide box, polypropylene

Dupont NEN Thomas Thomas Fisher Sigma Pharmco Sigma Sigma Sigma Thomas Eastman Kodak Eastman Kodak Eastman Kodak Fisher Fisher Thomas Nalge Nunc Nalge Nunc Nalge Nunc Sigma Sigma Fisher Thomas Thomas

NET-027A 6662-F55 8542-E30 08-812 C5312 111000190CSGL S1526 25,811-3 S5881 3483-K33 165 4433 146 4593 197 1746 SH30-500D SP15-100 6684-H61 177380 177437 177445 S5136 P5655 A408-1 6707-M27 6708-G08

freeze–thaw will very rapidly decrease activity. The bulk trypsin solution should be thawed only once, dispensed in 5-mL portions in sterile 15-mL centrifuge tubes and then stored at –20°C until use. 3. Trypsin solution (0.25%): Five milliliters of sterile sodium bicarbonate (7.5%) is added to 40 mL of ice-cold Ca2+/Mg2+-free medium. Subsequently, 5 mL of freshly thawed 2.5% trypsin stock is added to the solution. This solution should be prepared just before the cells are treated and should be kept on ice.

2.4. Thymidine Incorporation Suppliers and more detailed information on the items required for measurement of thymidine incorporation are listed in Table 6. 1. [3H-methyl]-thymidine stock solution: Under sterile conditions, [3H-methyl]-thymidine (2 Ci/mmol, 1 mCi/mL) is diluted to a concentration of 5 µCi/mL in ster-

Fibroblast Model for Cell Senescence Studies

37

ile medium. This stock solution is aliquoted (5-mL portions) in a laminar flow hood using sterile procedures into sterile, 15-mL centrifuge tubes and stored at –20°C until use. 2. Phosphate-buffered saline (PBS) solution: dissolve 8 g of NaCl, 0.2 g of KCl, 1.44 g of Na2HPO4, and 0.24 g of KH2PO4 in 900 mL of H2O with magnetic stirring. The pH is adjusted to 7.4 with HCl, the volume adjusted to 1 L, and the solution is autoclaved for 20 min at 121°C. 3. Emulsion: Kodak NTB-2 emulsion is purchased in a lightproof container. The emulsion is stored at 4°C (see Note 6). 4. Developer and Fixer a. Kodak D-19 developer is purchased in packets that make 1 gal when reconstituted. The entire packet is used at one time and the solution is stored in a brown bottle in the dark. The developer remains useable for 1–3 mo. When the developer turns yellow, it is discarded. b. Acid fixer is made and stored in the same manner as the D-19 developer.

3. Methods 3.1. Cell Propagation in Serum-Supplemented Medium Cells may be grown in a variety of culture vessels (see Note 7). Amounts described in the following procedure are for a T-75 flask. Proportional amounts are used for other size vessels; i.e., for a T-25 flask, one third of all of the amounts given is used. Trypsinization and seeding of flasks should be performed in a sterile environment (see Note 8). To propagate adherent cells: 1. Prepare fresh trypsin solution (0.25%) and place it on ice; prepare fresh growth medium and warm it to 37°C. 2. Using sterile procedures in a laminar flow hood, remove spent growth medium from the culture vessel. For flasks and bottles, the medium should be removed by aspiration or decanting from the side opposite the cell growth surface. For cell culture plates and dishes, the medium should be removed by aspiration from the edge of the growth surface. 3. Gently wash the monolayers of adherent cells twice with 0.25% trypsin solution (4 mL). 4. Remove residual trypsin solution by aspiration from the side opposite the cell growth surface (flasks) or from the edge of the growth surface (plates, dishes, and slides) as appropriate. 5. Add enough trypsin solution (0.25%) to wet the entire cell sheet (2 mL/T-75). 6. The culture vessel should be tightly capped to maintain sterility and placed at 37°C. 7. The cells will assume a rounded morphology as they are released from the growth surface. Detachment of the cells should be monitored using a microscope. As a general rule, detachment will be complete within 15 min. The trypsinization process may be speeded up by gently tapping the sides of the flask. Care should be

38

8.

9.

10.

11. 12.

13. 14.

15. 16.

Cristofalo, Volker, and Allen taken to not splash cell suspension against the top and sides of the flask, because this will lead to errors in the determination of the number of cells in the flask. When all of the cells have detached from the growth surface, as determined by inspection with a microscope, the flask is returned to the laminar flow hood. Complete medium with 10% v/v FBS is carefully pipeted down the growth surface of the vessel to neutralize the trypsin and to aid in pooling the cells. For a T-75 flask, 8 mL of complete medium is used. The final harvest volume is 10 mL. Cell clumps should be dispersed by drawing the entire suspension into a 10-mL pipet and then allowing it to flow out gently against the wall of the vessel. The process is repeated at least three times. The procedure is then repeated with a 5-mL pipet. Until the procedure becomes routine, a sample is withdrawn and examined under the microscope to ensure that a suspension of single cells has been achieved. During this process, the cells should be kept on ice to inhibit cell aggregation and reattachment. Using sterile procedures, remove an aliquot from the cell suspension, then dilute it into Isoton II in a counting vial. Typically, 0.5 mL of the cell suspension is diluted into 19.5 mL of Isoton II. Count the sample with a Coulter Counter. Calculate the number of cells in the harvest. Calculate the volumes of cell suspension and complete medium needed for new cell culture growth vessels. In most cases, cells are seeded at a density of 1 × 104 cells/cm2 of cell growth surface, and the total volume of cell suspension plus complete medium added to the culture vessels is maintained at 0.53 mL/cm2 of cell growth surface. In the laminar flow hood, add the calculated amounts of complete medium to new culture vessels. Dissolved CO2 in equilibrium with HCO3- is the principal buffer system of the medium, although serum also has some buffering capacity. Because CO2 is volatile, the gas phases in the flasks are adjusted to the proper pCO2 to maintain the pH of the medium at 7.4. Using sterile procedures in a laminar flow hood, a 5% CO2/95% air mixture is passed through a sterile, cotton-filled CaCl2 drying tube, through a sterile pipet, and into the gas phase of the cell culture flask with the growth surface down. As the gas mixture is flushed over the medium surface, the color of the medium will change from a dark red toward a red-orange. The flask is flushed until the medium no longer changes color. At this point, the gas above the medium is 5% CO2 and the pH of the medium is 7.4 (see Note 4). The flask is then tightly capped to prevent gas exchange with the outside environment. Cells grown in culture plates, dishes, and Lab-Tek® slides, which are not gas-tight, are not equilibrated with the gas mixture in this manner; instead they must be grown in incubators that provide a humidified, 5% CO2 atmosphere. The cell harvest is resuspended with 10-mL and 5-mL pipets, as before. Inoculate each culture vessel to a final density of 1 × 104 cells/cm2 of growth surface. Briefly flush the culture vessel a second time with the 5% CO2/95% air mixture to replace the CO2 lost when the vessel was opened. Cap the flask tightly and

Fibroblast Model for Cell Senescence Studies

39

incubate at 37°C. Periodically, examine the color of the medium to ensure that the seal is gas tight. 17. The cumulative population doubling level (cPDL) at each subcultivation is calculated directly from the cell count (see Note 7).

Example: One week after seeding a T-75 flask with the standard inoculum of 7.5 × 105 cells at a cPDL of 37.2, the cells are harvested. One doubling would yield 2 × 7.5 × 105 = 1.5 × 106 cells; two doublings would result in 4 × 7.5 × 105 = 3.0 × 106 cells; three doublings would yield 8 × 7.5 × 105 = 6.0 × 106 cells, etc. Thus, the population doubling increase is calculated by the formula: NH/NI = 2X or

[log10 (NH) – log10 (NI)]/Log10 (2) = X

where NI = inoculum number, NH = cell harvest number, and X = population doublings. The population doubling increase that is calculated is then added to the previous population doubling level to yield the cPDL. For example, if 9.1 × 106 cells were harvested, then the population doubling increase can be calculated from the expression: 9.1 × 106 cells = 2 (X) × 7.5 × 105 cells X log10 2 = log10 (9.1 × 106) – log10 (7.5 × 105) X = 3.6

The population doubling increase is added to the previous cPDL to give the new cPDL of the cell population. For this example, the new cPDL is 37.2 + 3.6 = 40.8. The end of the replicative life-span was defined by failure of the population to double after 4 wk in culture with 3 wk of consecutive refeeding.

3.2. Cell Propagation in Serum-Free Medium 1. Because undefined mitogens and inhibitors present in serum complicate the interpretation of cell growth response results, soybean trypsin inhibition solution should be used to stop trypsin instead of complete medium with 10% v/v FBS to wash and collect the cells from the growth surfaces of flasks. Otherwise, cells are released from the surface of their culture vessel exactly as described previously for propagation of cells in serum-supplemented medium (Subheading 3.1., steps 1–12). 2. Wash the cells to remove residual mitogens and trypsin inhibitor, rather than using them directly to inoculate the culture flasks: a. Under sterile conditions, the cells are pelleted by centrifugation at 75g for 5 min at 4°C.

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Cristofalo, Volker, and Allen

b. The centrifuge tubes are placed in ice, transferred to a laminar flow hood, the supernatant is removed, and the cells are resuspended in 10 mL of incomplete serum-free growth medium (Subheading 2.1.). c. Under sterile conditions, the cells are again pelleted by centrifugation, and after removal of the supernatant, the cells are resuspended in 10 mL of complete serum-free growth medium (Subheading 2.2.). 3. Determine the cell number with the Coulter Counter as before, using an aliquot of the cell suspension (0.5 mL) 4. Cells are then seeded exactly as described in Subheading 3.1., steps 13–17, except that serum-free cell growth medium is used.

3.3. Replicative Life-Span As noted previously, cells in culture exhibit a finite number of replications. At the end of their in vitro life-span substantial cell death occurs; however, a stable population emerges that can exist in a viable, though nondividing, state indefinitely (128). Furthermore, small subpopulations of cells may retain some growth capacity even after the vast majority of cells in a culture are no longer able to divide. As a practical matter, cultures of cells may be considered to have reached the end of their proliferative life-span when the cell number fails to double after 4 wk of maintenance in growth medium with weekly refeedings. The maximum proliferative capacity of the cells is determined as follows: When cell cultures are near the end of their proliferative life-span, at least four identical sister flasks are prepared. One flask is harvested each week. If the number of cells harvested is at least double the number inoculated, cells are subcultivated as usual. One of the sister flasks may also need to be harvested to provide enough cells for subcultivation into four flasks. If the number of cells harvested is not at least double the number inoculated, all of the sister flasks are refed by replacement of the spent medium with fresh complete medium and equilibration with 5% CO2/95% air mixture. This process is repeated three times. When cultures fail to double during this period, the culture may be considered to have reached the end of proliferative life or to be “phased out.”

3.4. Saturation Density Cultures are grown until the cells are densely packed and no mitotic figures are apparent. This usually requires from 7 to 10 d after seeding for early passage cells, and more than 9 d for later passage cells. To estimate the saturation density, these confluent and quiescent cells are then harvested and counted as described previously. 3.5. Microscopic Estimate of Cell Density It is often desirable to obtain an estimate of cell density without harvesting the cells. A stage micrometer is used to calibrate the eyepiece micrometer and

Fibroblast Model for Cell Senescence Studies

41

determine the diameter of the field of view for each objective and ocular lens used. The area of the field of view is calculated as Area = π r2, where r is the radius of the field of view. Scan the sample to ensure that the cells are uniformly distributed. Then count at least 400 cells using random fields. Since the standard deviation of a Poisson distribution is the square root of the number, 400 cells are counted. The square root of 400 (20) is 5%, which is the limit of statistical reliability for most biological work. Record the number of cells and the number of fields counted. The cell density is then calculated as follows: cell density = (no. of cells counted)/([no. of fields counted] · [area per field])

3.6. Thymidine Incorporation 3.6.1. Coverslips 1. Place coverslips in a clean, glass rack using forceps. 2. Lower the rack containing the coverslips into a solution of chloroform/95% ethanol (1:1) and allow to soak for 30 min. 3. Rinse the coverslips with deionized water. 4. Submerge the coverslips in a 95:5 solution of concentrated sulfuric acid (95%)/ concentrated nitric acid (70%), previously prepared in a fume hood and allowed to cool to room temperature. Soak the coverslips in this solution for 30 min. 5. Rinse the coverslips thoroughly in deionized water. 6. The rack containing the coverslips should then be lowered into a solution 0.2 M NaOH and allowed to soak for 30 min. 7. Remove the coverslips from the NaOH solution and rinsed at least three times in deionized water. 8. Remove the coverslips from the rack and allow to air-dry on lint-free disposable wipes. 9. When completely dry, bake the coverslips for 3 h at 180°C for sterilization.

3.6.2. Cell Slides 1. In a laminar flow hood, under sterile conditions, cells are harvested and counted in the usual manner. 2. Cells are seeded at a density of 1 × 104 cells/cm2 on Lab-Tek® slides or in cell culture dishes that contain coverslips (Subheading 3.6.1.). If using coverslips use sterile forceps to arrange them in the dish so that they do not overlap one another. 3. Immediately after seeding, the slides and dishes are placed in an incubator at 37°C in an atmosphere of 5% CO2/95% air. 4. Twenty-four hours later, add the stock solution of [3H-methyl]-thymidine (specific activity 2 Ci/mmol; Subheading 2.4.) to the cultures to a final concentration of 0.1 µCi/mL. 5. After 30 h (129), the labeling medium is removed, and cells are immediately washed twice with PBS (Subheading 2.4.), fixed in 100% methanol for 15 min,

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Cristofalo, Volker, and Allen

and air-dried. If cells are grown on coverslips, remove the coverslips from the dishes and place in a clean ceramic or glass rack using forceps prior to washing and fixing. If a Lab-Tek® slide is used, the plastic container and gasket must be removed prior to washing and fixing. These procedures should be done rapidly to limit damage to the cells. The cells must not be permitted to dry before they are fixed. 6. Mount coverslips with the cell surface up using mounting resin. Allow the resin to dry overnight.

3.6.3. Autoradiography 1. Remove the Kodak NTB-2 emulsion from storage at 4°C and place it in a warm room at 37°C. The emulsion will liquefy in 3–4 h. The emulsion may also be melted by placing it in a 40°C water bath in the dark for about 1–1.5 h. Do not shake the bottle because the resultant bubbles may cause irregularities in the final emulsion thickness. 2. In a dark room, the desired amount of emulsion is gently, but thoroughly mixed in a 1:1 ratio with deionized, distilled water. 3. Add 15–20 mL of the 1:1 emulsion/water solution to a container (a slide mailer works well for this) previously set up in a 40°C water bath in the dark. 4. Dip each slide individually into the slide mailer. One dip is sufficient to coat the slide. 5. Place each dipped slide in a standing (vertical) position in a wire test tube rack to drain off excess emulsion. The slides are allowed to dry for 30 min in the dark. 6. The dipped slides are placed into a slide box with a desiccant. The box is covered and sealed with black electrical tape. The box is placed inside a second light-tight container that also contains a desiccant and this is also sealed with electrical tape. 7. The container is placed at 4°C for 4 d.

Development of Cell Slides 1. Pour Kodak D-19 developer and acid fixer into large glass dishes. 2. Open the slide containers in a dark room (photo-safe light can be used), and remove the slides and place them in racks. 3. Place the slides in developer for 5 min. 4. Transfer the slides fixer for 5 min. 5. At this point, the room light may be turned on, if desired. Gently rinse the slides for 15 min in cold running water. The slides should next be lightly stained with Harris” modified hematoxylin stain to enhance nuclear visualization.

3.6.5. Staining Slides 1. Place the developed slides in staining dishes containing Harris” modified hematoxylin stain for 5–10 min. This amount of time is sufficient to produce light staining. 2. Drain slides in slide racks on paper towels.

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3. Rinse the slides continuously with deionized, distilled water until the excess stain is removed and then drain them on paper towels. 4. Excess emulsion should be wiped from the back of slides while they are still damp 5. Air-dry the slides.

3.6.6. Counting Labeled Nuclei 1. For ease in identifying the limits of individual chambers under the microscope, if Lab-Tek® slides are used, the stain between the individual chambers can be removed with the end of a paper clip or push pin. 2. Silver grains over nuclei where [3H-methyl]-thymidine has been incorporated into the DNA will be readily visible at 400× magnification. Nuclei with five or more grains are considered labeled. 3. To determine the percentage of labeled nuclei, at least 400 cells are counted per coverslip or chamber using random fields. Typically, determinations are done in duplicate.

4. Notes 1. It is important that the highest quality deionized, distilled water is used to prepare growth medium and all other reagents used for cell culture. 2. It is important that the bottles not be filled to more than one half volume to prevent overflow during sterilization. 3. Prolonged heat destroys some medium components. 4. Cells in a culture environment require carbon dioxide for growth and survival, and we have found that well controlled CO2/bicarbonate buffered media gives superior growth when compared with media containing synthetic buffers, such as HEPES. 5. Insulin and IGF-I both stimulate growth through the IGF-I receptor, although insulin has lower affinity for the IGF-I receptor and 50-fold higher concentrations are required to achieve comparable growth. Insulin is less expensive than IGF-I, and despite the reduced specificity, insulin is satisfactory for most experiments. 6. Emulsion should never be stored near high-energy sources of radioactivity. 7. Cell cultures are typically subcultivated weekly. Multiple identical sister flasks are prepared at subcultivation, as a hedge against potential contamination or other anomalies. Because a substantial fraction (25–60%) of the cells do not survive subcultivation (130), the number of cells does not generally increase above the seeded cell number until approx 24 h after subcultivation. 8. Cultures should routinely examined microscopically for contamination, and tested for mycoplasma at 5-wk intervals (131).

Acknowledgments This work was supported by the National Institutes of Health Grants AG00378 and AG00532.

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108. Bell, E., Marek, L., Sher, S., Merrill, C., Levinstone, D., and Young, I. (1979) Do diploid fibroblasts in culture age? Int. Rev. Cytol. (Suppl. 10), 1–9. 109. Bayreuther, K., Francz, P. I., Gogol, J., Maier, M., and Meinrath, G. (1991) Differentiation of primary and secondary fibroblasts in cell culture systems. Mutat. Res. 256, 233–242. 110. Kontermann, K. and Bayreuther, K. (1979) The cellular aging of rat fibroblasts in vitro is a differentiation process. Gerontology 25, 261–274. 111. Bayreuther, K., Rodemann, H. P., Hommel, R., Dittamann, K., Albiez, M., and Francz, P. I. (1988) Human skin fibroblasts in vitro differentiate along a terminal cell lineage. Proc. Natl. Acad. Sci. USA 85, 5112–5116. 112. Campisi, J. (1997) Aging and cancer: the double-edged sword of replicative senescence. J. Am. Geriatr. Soc. 45, 482–488. 113. Livinstone, D., Eden, M., and Bell, E. (1983) Similarity of sister-cell trajectories in fibroblast clones. J. Cell Sci. 59, 105–119. 114. Linskens, M. H. K., Fseng, J., Andrews, W. H., Enlow, B. E., Saati, S. M., Tonkin, L. A., Funk, W. D., and Villeponteau, B. (1995) Cataloging altered gene expression in yong and senescent cells using enhanced differential display. Nucleic Acids Res. 23, 3244–3251. 115. Kumar, S., Vinci, J. M., Millis, A. J. T., and Baglioni, C. (1993) Expression of interleukin-1α and β in early passage fibroblasts from aging individuals. Exp. Gerontol. 28, 505–513. 116. Karlsson, C. and Paulsson, Y. (1994) Age-related induction of platelet-derived growth factor A-chain mRNA in normal human fibroblasts. J. Cell. Physiol. 158, 256–262. 117. Slayback, J. R., Cheung, L. W., and Geyer, R. P. (1977) Comparative effects of human platelet growth factor on the growth and morphology of human fetal and adult diploid fibroblasts. Exp. Cell Res. 110, 462–466. 118. Furth, J. J. (1991) The steady state levels of type I collagen mRNA are reduced in senescent fibroblasts. J. Gerontol. 46, B122-B124. 119. Wharton, W. (1984) Newborn human skin fibroblasts senesce in vitro without acquiring adult growth factor requirements. Exp. Cell Res. 154, 310–314. 120. Clemmons, D. R. (1983) Age-dependent production of a competent factor by human fibroblasts. J. Cell. Physiol. 114, 61–67. 121. Kondo, H. and Yonezawa, Y. (1995) Fetal-adult phenotype transition, in terms of the serum dependency and growth factor requirements, of human skin fibroblast migration. Exp. Cell Res. 220, 501–504. 122. Chen, Q., Fisher, A., Reagan, J. D., Yan, L. J., and Ames, B. N. (1995) Oxidative DNA damage and senescence of human diploid fibroblasts. Proc. Natl. Acad. Sci. USA 92, 4337–4341. 123. Venable, M. E., Lee, J. Y., Smyth, M. J., Bielawska, A., and Obeid, L. M. (1995) Role of ceramide in cellular senescence. J. Biol. Chem. 270, 30701–30708. 124. Tresini, M., Mawaldewan, M., Cristofalo, V. J., and Sell, C. (1998) A phosphatidylinositol 3-kinase inhibitor induces a senescent-like growth arrest in human diploid fibroblasts. Cancer Res. 58, 1–4.

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125. Ogryzko, V. V., Hirai, T. H., Russanova, V. R., Barbie, D. A., and Howard, B. H. (1996) Human fibroblast commitment to a senescence-like state in response to histone deacetylase inhibitors is cell cycle dependent. Mol. Cell. Biol. 16, 5210–5218. 126. Goldstein, S., Moerman, E. J., Soeldner, J. S., Gleason, R. E., and Barnett, D. M. (1979) Diabetes mellitus and genetic prediabetes. Decreased replicative capacity of cultured skin fibroblasts. J. Clin. Invest. 63, 358–370. 127. Phillips, P. D. and Cristofalo, V. J. (1988) Classification system based on the functional equivalency of mitogens that regulate WI-38 cell proliferation. Exp. Cell Res. 175, 396–403. 128. Matsumura, T., Zerrudo, Z., and Hayflick, L. (1979) Senescent human diploid cells in culture: suvival, DNA synthesis and morphology. J. Gerontol. 34, 328–334. 129. Cristofalo, V. J. and Sharf, B. B. (1973) Cellular senescence and DNA synthesis: thymidine incorporation as a measure of population age in human diploid cells. Exp. Cell Res. 76, 419–427. 130. Cristofalo, V. J. and Kritchevsky, D. (1965) Growth and glycolysis in the human diploid cell strain WI-38. Proc. Soc. Exp. Biol. Med. 118, 1109–1113. 131. Levine, E. M. (1972) Mycoplasma contamination of animal cell cultures: a simple, rapid detection method. Exp. Cell Res. 74, 99–109.

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3 Human T-Cell Clones Graham Pawelec 1. Introduction Techniques for generating human T-cell clones (TCCs) were first described nearly two decades ago (1,2). This was a direct consequence of the discovery of T-cell growth factor and the subsequent ability to propagate T-cells over extended periods (3). Early on, numerous publications in immunology indicated an apparently unlimited growth potential of normal mammalian T lymphocyte cultures; however, even at this time, other investigators challenged this conclusion (4,5). Nonetheless, the possibility remained that at least some TCCs represented an exception to the rule of the Hayflick Limit for growth of normal somatic cells. If this were the case, the real relevance of replicative senescence as a universal phenomenon would be highly questionable. On the other hand, if those T-cells surviving apparently indefinitely were endowed with the properties of stem cells rather than differentiated cells, this quandary would be resolved. However, as far as could be judged, the apparently immortal TCCs described in the literature seemed to possess all the attributes of normal T-cells, not stem cells. Several explanations for this apparent paradox have been proposed, the most likely of which may be that such immortal lines are in fact abnormal. Few clones were tested for karyotypic or other abnormalities. Such analyses, when performed, often revealed genetic aberrations in human as well as murine clones (6,7). In the case of murine cells, continuous cultures often transform spontaneously in culture, but in humans this is rare or absent. We and others have systematically approached the question of longevity of normal human TCCs using variations of the original interleukin-2 (IL-2)dependent cloning and propagation protocol (1,2). This procedure involves limiting dilution of the cell suspension to be cloned, and microwell culture of the

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diluted cells on an irradiated feeder cell layer in the presence of chemically defined media supplemented with growth factors (e.g., IL-2). We wished to establish whether culture aging of TCCs did occur, how it could be characterized, and whether it depended on the source and nature of the T-cells studied. The first question approached was to what extent culture conditions affected cloning efficiencies and longevity of the TCC (8); most of our early data were then obtained using a standard culture system employing medium supplemented with human serum (HS) and natural IL-2 (9) and using feeder cells consisting of a pool of peripheral blood mononuclear cells (PBMCs) from >20 random healthy donors. The T-cells to be cloned were derived from young adult donors and were mostly prestimulated with alloantigens. Under these conditions, the type of T-cell predominantly derived was CD4+ and carried the α/β T-cell receptor (TCR2). This chapter therefore focuses on this type of TCC, and not on CD8 or TCR1 cells, which seem to behave somewhat differently in culture, but which we have not studied so extensively. For meaningful studies of T-cell aging in vitro, it is essential to know the in vivo age of the starting T-cell population. This is impossible in a mixture of T-cells from an adult individual, as separation of subsets according to naive cell and memory cell markers is a crude and inaccurate method. Assessing age by measuring telomere lengths, even of individual cells, is also not satisfactory, as it cannot take into account whether telomerase has been activated at some point in these cells (10). The only way to be sure that all T-cells being studied are of the same age at the beginning of the experiment is to isolate precursors and cause them to differentiate into T-cells in vitro (11,12). Longevity comparisons between these pre-T-cell-derived precursors and TCCs from mature T-cells of the same donors suggested that the latter have a shorter life expectancy corresponding to the time required for the precursors to develop into T-cells in vitro (13). We therefore hypothesized that the T-cell “clock” was first set at the time when fully mature T-cells were generated and not at some time beforehand at the precursor or stem cell level (14). Going further back in the T-cell differentiation pathway, CD34+ stem cells have the potential to develop into T-cells in in vitro culture systems (15), and this property could be exploited to study T-cell aging. Thus far, cumbersome thymic organ culture or thymic stromal cell culture systems have been required for this; here we present a variant TCC culture protocol that allows the generation of mature T-cells from isolated CD34+ stem cells in liquid culture in the absence of thymic components. 2. Materials 1. T-cell growth factors (TCGFs): The quality and purity of the T-cell growth factors employed is critical. The main and most commonly used TCGF is IL-2 in the form of purified recombinant protein. Nowadays, many companies offer high-

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quality IL-2; the investigator should pretest a batch for suitability for the cells being cultured. Mixtures of TCGFs may be useful in some circumstances, especially IL-2 + IL-4 or IL-2 + IL-7 (see Subheading 3.). Major suppliers are Genzyme, Endogen, PeproTech, R and D Systems, Boehringer-Mannheim, and so on. 2. Monoclonal antibodies (MAbs): Again, there are many suppliers of different antibodies, and companies favored will be different in different parts of the world. Major suppliers are Becton-Dickinson, Coulter-Immunotech, DAKO, and so on. However, for certain purposes, such as cell isolation with magnetic beads (see Subheading 2.3.1.), it may be economically desirable to obtain hybridoma cells and produce MAb oneself. The hybridoma cells are easy to grow, and for the purposes of cell separation, culture supernatants do contain enough MAbs. Obtaining hybridomas may be a problem, but cell banks such as the American Type Culture Collection (ATCC) can provide hybridomas secreting MAbs against common antigens sufficient for most cell separation purposes. 3. Magnetic beads: a. Dynabeads (Dynal, Oslo, Norway): In this method, the T-cell population is negatively selected after the cells are labeled with cocktails of MAbs against B cells (e.g., CD19), natural killer (NK) cells (CD16), monocytes (CD14), major histocompatibility class (MHC) II, etc. Because of the amount of MAbs required it is recommended that hybridoma supernatants are used. Dynabeads M450 coated with sheep anti-mouse IgG can be employed for most negative cell separations. b. Miltenyi CD34 Progenitor Kit (Miltenyi Biotec, Bergisch-Gladbach, Germany) is supplied with the necessary reagents for CD34 cell isolation by positive selection. The magnetic particles are precoated with CD34 MAbs directed against a particular exposed epitope of CD34 (class II epitope); purity of the derived population can then be checked with a MAb directed at a different epitope (e.g., anti-class I MAb My10 from Coulter-Immunotech). 4. Culture medium: Human serum for supplementing media such as RPMI 1640 or IDMEM to support long-term growth of T-cells cannot be reliably obtained commercially. It is necessary to prepare and screen the serum on T-cells in the laboratory. Serum can be obtained or purchased from blood banks, but it is difficult to obtain enough male nontransfused AB donors for regular use. It may be satisfactory to use male nontransfused donors of any blood type, as we do. Serum is separated from coagulated blood by centrifugation and one aliquot from each of at least 20 sera prepared at the same time is heat inactivated (30 min at 56°C). The bulk of the sera are frozen separately. The test samples are then separately tested for their ability to support T-cell proliferation. Those that are judged satisfactory are then thawed and pooled. Culture medium must be supplemented with 10–20% of such serum pools to support long-term T-cell growth. Alternatively, use X-Vivo 10 or X-Vivo 15 serum-free medium (BioWhittacker), formula unknown.

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3.1. Source of Cells to Be Cloned 3.1.1. Purification of T Cells (see Note 1) 1. Selectively deplete non-T-cells from PBMC using a cocktail of antibodies for CD14 (expressed by monocytic cells, macrophages, dendritic cells), CD16 (on NK cells), CD19 (B cells and B-cell precursors), and HLA-DR (monocytes, B cells, activated T-cells, and dendritic cells). 2. Incubate 107 cells/mL at 4°C for 30 min with approx 10 µg/mL of each antibody, centrifuge, wash twice, and resuspend in 1.5 mL of phosphate-buffered saline (PBS) with 0.1% bovine serum albumin (BSA). 3. Add approx 108 washed Dynabeads in 0.5 mL and incubate at room temperature for 1 h. Gently shake occasionally. Add 2 mL of PBS and put the tube into the magnetic field for 1–2 min. Gently aspirate the supernatant. This contains the negatively selected cells not held by the magnet. Wash twice and control purity with anti-T-cell antibody by immunofluorescence. 4. Remove any possible remaining functional accessory cells by treating the population with L-leucyl-L-leucine methyl esther (LME). Incubate at 2.5 × 106/mL in 10 mM LME for 45 min at room temperature in culture medium without serum. 5. Wash twice and then check absence of functional accessory cells. This can be done by stimulating with T-cell mitogens such as phytohemagglutinin in the absence of added cells. There should be no response. After reconstitution of accessory function with B-lymphoblastoid cell lines (B-LCLs) the response should be measurable. A typical protocol is to incubate 2.5 × 104 T-cells per round-bottom microtiter plate well in culture medium together with 1% phytohemagglutinin (PHA, M form; Gibco-BRL) in triplicate. A duplicate set of wells receives in addition 2.5 × 104 B-LCL cells (irradiated at 80 Gy). Proliferation can be assessed 3 d later, for example, by addition of 37 kBq/well of tritiated thymidine and assessing incorporated nuclear radioactivity after 8–16 h.

3.1.2. Purification of CD34+ Cells 1. Separate low-density mononuclear cells (MNCs, one third full) to 7 mm diameter flat-bottom microtiter plate wells with fresh medium and 1 × 105 feeder cells. Retain Terasaki plates for up to 2–3 wk and examine again at intervals to identify any late positive wells. Transfer these to microtiter plates as well. 5. Examine microtiter plates every few days. Split those becoming overcrowded with growing cells 1:1 into new culture wells and re-feed with medium (but not feeder cells). After about a week in microtiter plates, transfer contents of wells with growing cells into 16 mm diameter cluster plate wells with 2–5 × 105 feeder cells, and fresh medium (see Note 4). 6. Observe after 3–4 d. Divide wells that are full or nearly full into four, the others into two, with fresh media, but no more feeder cells. After a total of about a week in cluster plates, count the number of cells in each clone and subculture to 2 × 105/well, again with 2–5 × 105 feeders/well and fresh medium. Supplement with fresh medium after 3–4 d and subculture again if necessary. Continue to propagate by

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weekly or fortnightly subculture with new feeder cells and fresh medium (see Note 5). 7. Estimate longevity in terms of population doublings (PDs). Score initial limiting dilution cloning wells as positive if at least one third full of growing cells. One third of the surface of a Terasaki well is equivalent to about 1000 cells (= 10 PDs). Use the number of clones derived at this stage to calculate the average life-span of all clones derived, that is, do not include clones achieving less than this number of PD in the analysis. The number of cells per microtiter plate well prior to cluster plate transfer is approx 1 × 105 (= approx 17 PDs). Assume that clones dying between the Terasaki and microtiter plate stages have undergone 17 PDs, and use this figure in calculations of average longevity. After clones are transferred to 16 mm diameter cluster wells, the number of PDs undergone can be estimated for each clone from the exact number of cells counted at each subculture. Cryopreserve cells at any point of their life-spans. Continue calculations of PDs undergone on the basis of the number of viable cells replated after thawing, not the number of cells originally present. Take maximum life-span of cells in each cloning experiment to be the PDs corresponding to the time point of the death of the longest living clone in each case (see Note 6).

4. Notes 1. For many applications where mature T-cells are to be cloned, it is not necessary to purify them beforehand. PBMCs as the starting population can be so stimulated that only T-cells can grow (e.g., with T-cell mitogens or antigens). When using CD34+ cells, purity is, however, critical, because contaminating non-CD34 cells most likely have a growth advantage over the CD34+ cells. 2. Clearly the culture medium employed is a critical aspect of the technique. For many years, we and others found that although T-cells could be grown for limited periods in completely chemically defined serum-free media, cloning and long-term propagation in such media was not possible. We and others were forced to use a serum supplement, most commonly FCS or HS. In our experience, very few batches of FCS prove suitable for human T-cell cloning and extensive propagation. Unfortunately, the same was true for commercially available HS. We have therefore always obtained material from blood banks and prepared the serum ourselves. Because of paucity of AB blood donors, we have always used nontransfused male blood. Each serum is separately heat inactivated (56°C, 30 min) and individually tested for its ability to support lymphocyte proliferation. Sera supporting acceptable levels of proliferation (usually around 80% of tested samples) are pooled and used at 10– 20% v/v with culture medium (RPMI 1640 or IMDMEM). However, note that CD34+ cells cannot be grown in either FCS- or HS-containing medium. Two factors recently enabled us successfully to grow these cells and facilitate their differentiation into mature T-cells. The first was to use the serum-free medium X-Vivo 10 without adding any other serum supplement. This medium is also suitable for the cloning and long-term propagation of TCC derived

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4.

5.

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from mature T-cells. The second factor was to use a suitable cytokine cocktail that supported the viability of the stem cells and also allowed T-cell growth to take place. This consisted of stem cell factor (SCF), flt3-L, and IL-3, together with IL-2 and either oncostatin M (OM) or IL-7. Use autologous PBMCs, a mixture of autologous PBMCs and autologous B-lymphoblastoid line cells, or other appropriate antigen-presenting cells (APC), in the presence of specific antigen. Alternatively, use an antigen-non-specific stimulus such as 50 ng/mL of the anti-CD3 monoclonal antibody OKT3 or 2 µg/mL of purified or 1% crude PHA, together with the same number of allogeneic or autologous PBMCs, or pooled PBMCs (irradiated at 30 Gy). Clones successfully propagated in cluster plate wells for 2 wk can be taken to be established. They can at this point be cryopreserved, although it is advisable to retain some of each clone in culture to test different conditions to establish optimal parameters for each particular clone. Human TCCs can be readily cryopreserved using the same protocols as are suitable for freezing resting T-cells. Having a frozen stock enables the different culture conditions to be tested to optimize growth, without risking the loss of the whole clone. Restimulation parameters should be established for each clone. T-cells require periodic reactivation through the T-cell antigen receptor to retain responsiveness to growth factors. This can be accomplished either specifically or nonspecifically. All clones can be propagated with weekly restimulation; some but not all can be propagated with restimulation only every 2 wk. It should be established whether each clone can be propagated with the most convenient feeder cells (80 Gy-irradiated B-LCL) instead of PBMC feeders. Most TCCs flourish on B-LCLs alone, but some need the presence of PBMCs as well (this is especially true during cloning). Propagation of the TCCs on PBMC feeders can also be continued, but for practical reasons it may often be more convenient and easier to grow large amounts of B-LCLs than to isolate the PBMCs. For convenience, it is also easier to grow TCCs in scaled-up culture vessels than in 16 mm-diameter culture wells. However, not all clones can be adapted to growth in flasks. This has to be tested for each clone, using between 1 × 105 and 5 × 105/mL of TCCs with an equal number of feeders in tissue culture flasks. Clones not growing under these conditions can rarely be adapted to growth in flasks by altering the amounts or concentrations of TCCs or feeders or by increasing or decreasing the frequency of stimulation and/or feeding. Longevity estimation in PD is an extremely conservative measurement indicating the absolute minimum number of cell divisions achievable by each cell. This is because it simply assumes that all daughter cells at each cell division are viable and themselves capable of dividing and generating two viable progeny. In reality, it is highly likely that this is not the case.

Acknowledgments Work in the author’s laboratory is supported by the Deutsche Forschungsgemeinschaft, the Dr. Mildred Scheel Foundation, the Dieter Schlag Founda-

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tion, the VERUM Foundation, the Novartis Foundation for Gerontological Research, the ƒortüne Program of University of Tübingen Medical Faculty, and the European Commission (see http://www.medizin.uni-tuebingen.de/ eucambis/). References 1. Bach, F. H., Inouye, H., Hank, J. A., and Alter, B. J. (1979) Human T-lymphocyte clones reactive in primed lymphocyte typing and cytotoxicity. Nature 281, 307–309. 2. Pawelec, G. and Wernet, P. (1980) Restimulation properties of human alloreactive cloned T cell lines. Dissection of HLA-D-region alleles in population studies and in family segregation analysis. Immunogenetics 11, 507–519. 3. Morgan, D. A., Ruscetti, F. W., and Gallo, R. C. (1976) Selective in vitro growth of T-lymphocytes from normal human bone marrows. Science 193, 1007–1008. 4. Effros, R. B. and Walford, R. L. (1984) T cell cultures and the Hayflick limit. Hum. Immunol. 9, 49–65. 5. Pawelec, G. (1985) Functions and changing activities of interleukin 2-dependent human T lymphocyte clones derived from sensitization in mixed leukocyte cultures, in T Cell Clones (von Boehmer, H. and Haas, W., eds.), Elsevier, Amsterdam, Holland, pp. 311–322. 6. Johnson, J. P., Cianfriglia, M., Glasebrook, A. L. and Nabholz, M. (1982) Karyotype evolution of cytolytic T cell lines, in Isolation, Characterisation, and Utilisation of T Lymphocyte Clones (Fathman, C. G., Fitch, F. W., eds.), Academic Press, New York, pp. 183–191. 7. Kaltoft, K., Pedersen, C. B., Hansen, B. H., and Thestrup-Pedersen, K. (1995) Appearance of isochromosome 18q can be associated with in vitro immortalization of human T lymphocytes. Cancer Genet. Cytogenet. 81, 13–16. 8. Kahle, P., Wernet, P., Rehbein, A., Kumbier, I., and Pawelec, G. (1981) Cloning of functional human T lymphocytes by limiting dilution: impact of feeder cells and interleukin 2 sources on cloning efficiencies. Scand. J. Immunol. 4, 493–502. 9. Pawelec, G., Schwuléra, U., Blaurock, M., Busch, F. W., Rehbein, A., Balko, I., and Wernet, P. (1987) Relative cloning efficiencies and long-term propagation capacity for T cell clones of highly purified natural interleukin 2 compared to recombinant interleukin 2 in man. Immunobiology 174, 67–75. 10. Effros, R. B. and Pawelec, G. (1997) Replicative senescence of T lymphocytes: does the Hayflick Limit lead to immune exhaustion? Immunol. Today 18, 450–454. 11. Pohla, H., Adibzadeh, M., Buhring, H. J., Siegels-Hubenthal, P., Deikeler, T., Owsianowsky, M., Schenk, A., Rehbein, A., Schlotz, E., Schaudt, K., and Pawelec, G. (1993) Evolution of a CD3+CD4+ alpha/beta T-cell receptor+ mature T-cell clone from CD3-CD7+ sorted human bone marrow cells. Dev. Immunol. 3, 197–210. 12. Preffer, F. I., Kim, C. W., Fischer, K. H., Sabga, E. M., Kradin, R. L., and Colvin, R. B. (1989) Identification of pre-T cells in human blood. Extrathymic differentiation of CD7+CD3- cells into CD3+ gamma/delta or alpha/beta + T cells. J. Exp. Med. 170, 177–190.

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13. Adibzadeh, M., Pohla, H., Rehbein, A., and Pawelec, G. (1995) Long-term culture of monoclonal human T lymphocytes: models for immunosenescence? Mech. Ageing. Dev. 83, 171–183. 14. Pawelec, G., Rehbein, A., Haehnel, K., Merl, A., and Adibzadeh, M. (1997) Human T cell clones as a model for immunosenescence. Immunol. Rev. 160, 31–43. 15. Freedman, A. R., Zhu, H. H., Levine, J. D., Kalams, S., and Scadden, D. T. (1996) Generation of human T lymphocytes from bone marrow CD34+ cells in vitro. Nat. Med. 2, 46–51.

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4 Telomeres and Replicative Senescence Hector F. Valenzuela and Rita B. Effros 1. Introduction Telomere length measurement can be used both to monitor the proliferation of long-term cultures of somatic cells as well as to determine the replicative history of in vivo-derived cells. The most frequently used technique for telomere length measurement is Southern hybridization (1,2). The method consists of isolating total genomic DNA, digesting the DNA with restriction enzymes so as to isolate the undigested telomere restriction fragments (TRFs), and separating these fragments by gel electrophoresis. The DNA is denatured and transferred from the gel to a membrane or filter, and the DNA samples are then hybridized to radiolabeled complementary probe. However, when blotting TRF DNA to the membrane, differential transfer may occur owing to inefficient transfer of larger fragments of DNA (>10 kb) to a membrane. As the mean length of the TRF is based on the assumption that the amount of telomeric DNA (TTAGGG repeats) in a given TRF is proportional to the length (3,4), this would lead to possible error in calculating the mean length of the telomeres. The method that we present here avoids these potential problems by eliminating the membrane blot step altogether and probing the gel directly. The following protocol has been refined for measuring telomeric DNA length from human cells. Similar protocols can be adapted to measure telomere lengths in cells from other species. However, researchers should adjust the probe sequence for hybridization (not all species have the same telomere sequence) and optimize the restriction enzymes to obtain TRF within the resolvable molecular weight range of the gel because some species may have extremely long telomeres. For more information regarding telomeres, we suggest reading Kipling’s The Telomeres (5).

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The TRF assay method that we present here can be divided into three stages: (1) isolation and digestion of genomic DNA; (2) gel electrophoresis, drying, and hybridization; and (3) analysis of TRF length. We will emphasize in this protocol the measurements of the TRF after the DNA isolation. As mentioned previously, the protocol improves on the standard Southern blot procedure by eliminating the DNA transfer from gel to membrane, thereby reducing the time and labor involved. After digestion, the DNA fragments are separated in an agarose gel, which is then dried. The gel is then denatured, neutralized, and hybridized in a manner similar to the membrane in the usual Southern blot method. Once the gel is washed, it can be analyzed directly by densitometry of an autoradiograph or by using a phosphorimager (3). 2. Materials 1. Denaturing buffer solution: 1.5 M NaCl, 0.5 M NaOH. Dissolve 43.83 g of NaCl and 10 g of NaOH in 400 mL of distilled water, then raise volume to 500 mL. Store at room temperature. 2. Neutralization buffer solution (1.5 M NaCl, 0.5 M Tris-Cl): Dissolve 43.83 g of NaCl and 39.4 g of Tris-Cl in 450 mL of distilled water. Adjust pH to 8.0 (with approx 2 g of NaOH). Raise volume to 500 mL. Store at room temperature. 3. Hybridization buffer solution: Mix 64 mL of distilled water, 25 mL 20× saline sodium citrate (SSC), 10 mL of Denhardt’s reagent (50×), and 1 mL of sodium pyrophosphate (stock 1 M). Sterile filter with 0.22 µm filter. Store at 4°C. The 50× Denhardt’s reagent (Sigma Chemical, St. Louis, MO, USA) can be prepared using 5 g of bovine serum albumin, 5 g of Ficoll, and 5 g of polyvinylpyrrolidine. 4. 20× SSC washing solution (3 M NaCl, 0.3 M sodium citrate): Dissolve 175.3 g of NaCl and 88.2 g of sodium citrate in 800 mL of H2O. Adjust pH to 7 with a few drops of NaOH. Raise volume to 1 L with distilled water. Prepare 1.5L of 0.5× SSC for washes. Sterilize by autoclaving, and store at room temperature. The solution is stable for several months. 5. Probe: The probe sequence can be either (TTAGGG)3 or (CCCTAA)3. Prepare an aliquot concentration of 40 pmol/µL. Store at –20°C (see Subheading 3., step 4). 6. Ladders: A ladder that ranges from 1 to 20 kb is required. Alternatively, we advise mixing two ladders, a 1 kb ladder at 1 µg/µL (Gibco-BRL) and λDNA HindIII digest at 1 µg/µL (New England BioLabs). Store at –20°C. 7. Enzymes: Restriction enzymes HinfI and RsaI (New England BioLabs). Use appropriate buffer to ensure maximum activity. Klenow polymerase (Gibco-BRL) and T4 polynucleotide kinase (Gibco-BRL) are used for labeling ladder and probe, respectively. 8. Quick Spin Columns Sephadex G25 Fine (Boehringer Mannhein cat. no. 1273949). 9. Polyethylene bags for hybridization step. (Fisher bags cat. no. 01-812-10E; Fisher International Headquarters, 50 Fadem Rd, Springfield, NJ 07081-3193, USA: Tel: 201-467-6400; Fax: 201-379-7415).

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10. Isotopes: require [α-32P]ATP and [γ-32P]ATP for labeling ladder and probe, respectively. Alternatively one can use 33P isotopes, but do not mix these different isotopes in the same gel. 33P is safer to handle but requires twice the exposure time of 32P. 11. Gel apparatus: Gel cast of 15–20 cm long and thin combs (2 mm). 12. Whatman 3MM paper cut out to 2.5 cm longer than gel in both length and width.

3. Methods 1. Isolation of genomic DNA: The isolation of the genomic DNA can be performed by any number of standard protocols in the literature. We recommend “DNAzol” DNA Isolation reagent (Molecular Research Center, Inc., 5645 Montgomery Road, Cincinnati, OH 45212, USA; Tel: 513-841-0900; Fax: 513-841-0080). The main consideration in selecting a protocol should be to choose a method that yields DNA fragments larger than 60 kb; otherwise results may be skewed (see Note 1). 2. Digestion of genomic DNA: For the digestion of 10 µg of high molecular weight DNA use the following recipe: 10 µg of Genomic DNA 10 µL of 10× reaction buffer 2 µL of HinfI 2 µL of RsaI X µL dH2O 100 mL (final volume). Incubate at 37°C for 2–3 h. Run 2 mL of digested DNA with undigested DNA on a mini-gel to test for completion of digestions. Digestion is incomplete if there are fragments larger than 50 kb. The amount of DNA loaded per lane must be at least 1–2 µg; a larger amount of DNA increases the sensitivity of detection, especially for short telomeres. 3. Agarose gel: Pour a 0.5% agarose/0.5× TBE gel. The gel must be at least 10 cm long (we recommend 15–20 cm) and must be approx 3/4 cm thick. The longer gel allows good separation of large fragments of DNA and the thin combs prevent the DNA from diffusing. Run gel for a total of 750 V/h. Do not run gel faster than 50 V, as this prevents good resolution of long telomere fragments. We recomend 30 V, which should then be run for 25 h (for a total of 750 V/h). 4. Loading DNA onto gel: Load at least 1–2 µg of DNA per lane. Labeling of 1 kb and λDNA HindIII digest ladders is performed as follows in a 1.5-mL Eppendorf tube. Ladders should be loaded last onto the gel to minimize exposure to radiation. 0.5 µL of 1 kb ladder 0.5 µL of λDNA HindIII digest ladder 4 µL of 10× Klenow buffer 3 µL of [α-32P]ATP 31 µL dH2O (or add dH2O to 40 µL final volume) 1 µL Klenow fragment 40 µL (final volume)

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Fig. 1. Setup for drying the agarose gel in a gel dryer. Incubate 2 min at 37°C, and put on ice while you prepare the quick spin column to remove the unincorporated [α-32P]ATP. For a freshly prepared ladder use 0.5 µL/lane (see Notes 2 and 3). 5. Gel drying: Place two sheets of 3MM Whatman chromatography paper in gel drier, then place gel on top leaving 2.5 cm around the margins of the gel. Finally, place Saran wrap over gel and Whatman paper (Fig. 1). Dry gel under vacuum at 60°C for 45–75 min. Note: Start preparing the probe (step 7) and prewarm hybridization buffer (step 8) (see Note 4) 6. Gel washes: Remove gel from vacuum dryer by holding gel at the opposite end from the wells. Gels occasionally tear during this step, and by handling the end furthest from the wells, the chance of tearing the gel in the area near the DNA is reduced. One should keep in mind that at this point the gel is radioactive from the labeled ladder, so all proper precautions should be maintained. If Whatman paper sticks to gel, use water to peel them apart. Place the gel in a Pyrex dish container and add enough denaturing reagent (500 mL) to completely submerge gel. Let sit at room temperature for 10 min with gentle shaking. Dispose of buffer by pouring into proper radioactive waste. Repeat washing with neutralization buffer (500 mL) in the same Pyrex container for 10 min at room temperature with gentle shaking. Upon completion of this wash, dispose of buffer in the same manner. 7. Probe labeling reaction can be prepared the following way in a 1.5-mL Eppendorf tube: 1 µL of 40 pmol/µL of (TTAGGG)3 oligo 5.3 µL of 10× T4 polynucleotide kinase buffer 6.5 µL of [γ-32P]ATP 1.5 µL of T4 polynucleotide kinase 38.7 µL of dH2O 53 µL (final volume)

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9. 10.

11.

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Incubate reaction at 37°C for 30 min, then use a quick spin column to separate unincorporated [γ-32P]ATP. Hybridization of gel: Prewarm the hybridization buffer (15 mL) to 37°C. For a gel 10–20 cm in length use 15 mL (but no more than 20 mL) of hybridization buffer. Add 1 µL of label probe for every 1 mL of hybridization buffer. Keep at 37°C while you prepare the gel. Prepare the gel for hybridization by placing the gel in a hybridization bag. Perform this by cutting open two sides of the bag, so that it opens up like a book. This method will prevent the gel from sticking to plastic that may lead to tearing of the gel. Place gel inside and seal ends of plastic bag once again with an Impulse Sealer so that there is only one opening. (There should be 1–2 cm of margin space between the gel and the plastic bag.) Through this opening add the prewarmed hybridization buffer with the added probe. Check the seals for leaks over the Pyrex container before proceeding. Caution: The hybridization solution will be highly radioactive. Before sealing the open end of the bag, remove any bubbles that may be in the bag. (Remember to do this over a Pyrex container in case of leaks.) It may be helpful in removing the final bubbles to seal the bag at the very edge, then squeeze the remaining bubbles to one edge, sealing them off with a second seal. You must remove most of the bubbles before the second seal for this double sealing to work. Incubate the gel at 37°C for at least 6 h (although we recommend overnight). For very weak probes, 2–3 d may be necessary. Washing with SSC: Cut open plastic bag, remove the hybridization solution, and dispose in a proper radioactive waste receptacle. Carefully place gel in a Pyrex container and add 500 mL of 0.5× SSC (prewarmed at 37°C) for 6–7 min. Repeat the same wash two more times. Remove all SSC, then enclose the gel in Saran Wrap before exposing to imaging film (see Note 5). Analysis: Analysis of the TRF length can be done either by densitometric scanning of the autoradiogram or by using a phosphoimager (see Note 6)

4. Notes 1. For the isolation of genomic DNA, we recommend using a guanidium based method, such as DNAzol® (Molecular Research Center). This method is fast (30 min) and reliable; however, we emphasize the importance of using wide-mouth tips to prevent DNA shearing during the isolation steps. 2. Along with the samples, we recommend including two control DNA fragments, one from cells with long telomeres and the other from cells with short telomeres. For the long telomere source, one can use a subline of the 293 tumor, which has stable TRF length of approx 10.5 kb. As source of short telomeres, Daudi cells, with a mean TRF length of approx 3.9 kb, can be used. Prepare a large batch of genomic DNA from the control cell lines, aliquot into small amounts, and use these as standards to compare gel-to-gel variations in the sizes of TRF that may occur between experiments. It may be necessary to add more than 2 µg of DNA

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Valenzuela and Effros for cells that have short telomeres. Longer telomeres, because they have more TTAGGG repeats, give a stronger signal than short telomeres. The term “TRF length” is not synonymous with “telomere length.” The TRF includes both the telomere repeats and the adjacent subtelomeric region that contains both repetitive and nonrepetitive sequences . We suggest using ladders at both ends of the gel. Use 0.5 µL/lane of ladder labeling reaction mix when using freshly prepared material. If the ladder was prepared at an earlier time, add up to 1 µL/lane, but beware of adding too much lest the signal be overexposed, making it impossible to quantitate TRF length. If this problem occurs, one may either add more (>2 µg/lane) genomic DNA, or a smaller quantity of labeled DNA ladder per lane (but not less than 5 µg/lane). Gel orientation can be marked in a number of ways, such as by cutting away a corner of the gel, loading two lanes with the ladder on one side of gel, or loading control TRF DNA on one side of gel. Note that a 0.5% agarose gel is extremely fragile, and we suggest using a spatula or the back of a Pyrex dish during all transfers. Gels can be stored for up to 2 d at this stage, for example, if the probe is not ready. This can be done by placing the gel in a sealed plastic bag with 2 mL of 2× SSC at 4°C, as described in Subheading 3. If the background radiation is too high, place gel once for 1 h (or twice for 30 min each) at 48°C in 0.1× SSC. Shortening exposure time may also reduce background. If the problem continues, one should increase the amount of DNA, which reduces the noise by reducing the background. Adding lower concentrations of probe will also help reduce the background. If there is no signal, lengthen exposure time. Verify that probe was properly labeled. Verify that hybridization solution has no precipitates. If precipitate is seen, it is necessary to prepare fresh hybridization solution. The mean TRF length is calculated by integrating signal intensity over TRF distribution on gel as a function of mol wt. Divide a scanned TRF image into a grid in which columns cover the entire length of TRF sample analyzed and there are at least 30 boxes dividing each column (Fig. 2). The following equation can then be used to calculate the mean TRF length: Mean TRF length = Σ(ODi·Li) / Σ(ODi) where ODi is the intensity signal and Li is the mol wt at a particular (i) box in the grid as compared to a size marker from a ladder. Before using above equation for TRF length analysis, background must be subtracted from ODi. For each sample the background can be calculated by averaging the OD from the top two boxes and bottom two boxes adjacent to the smear. The average background OD is then subtracted from each box in the grid for that particular sample. Alternatively, some phosphorimagers (Packard, Instant Imager) come included with software that can quantitate the intensities of signals to obtain mean values that are plotted to preassigned values from a ladder.

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Fig. 2. Calculation of mean TRF length. Exposed image has been divided into columns and rows where OD and L values can be measured. For column 3, the boxes used for calculating the background are indicated.

Acknowledgments The authors wish to thank Dr. Choy-Pik Chiu (Geron Corporation, Menlo Park, CA) for providing comments on the manuscript, and the Geron Corpora-

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tion for their generous help in establishing the telomere assays in our laboratory. This work was supported by the National Institute on Aging (AG10415), the UCLA AIDS Institute, and the Seigel Life Project/UCLA Center on Aging. References 1. Southern, E. M. (1975) Detection of specific sequences among DNA fragments separated by gel electrophoresis. J. Mol. Biol. 98, 503. 2. Sambrook, J., Fritsch, E. F., and Maniatis, T. (1989) Molecular Cloning A Laboratory Manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, pp. 9.31–9.58. 3. Allsopp, R. C., Vaziri, H., Patterson, C., Goldstein, S., Younglai, E. V., Futcher, A. B., Greider, C. W., and Harley, C. B. (1992) Telomere length predicts replicative capacity of human fibroblasts. Proc. Natl. Acad. Sci. USA 89, 10,114–10,118. 4. Harley, C. B., Futcher, A. B., Greider, C. W. (1990) Telomere shorten during ageing of human fibroblasts. Nature 345, 458–469. 5. Kipling, D. (1995) The Telomere, Oxford University Press, pp. 1–12, 78–96, 130–142, 146–163.

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5 Detection of Molecular Events During Apoptosis by Flow Cytometry Ruaidhri J. Carmody,* Ana P. Costa-Pereira,* Sharon L. McKenna, and Tom G. Cotter 1. Introduction Apoptosis describes an intrinsic cell suicide program that may be activated by both endogenous and exogenous stimuli. This method of cell death is characterized by specific morphological features including chromatin condensation, nuclear fragmentation, cell shrinkage, membrane blebbing, and the formation of membrane-bound vesicles termed apoptotic bodies (1). Apoptosis has come to be referred to as the physiological mode of cell death, as it allows cellular destruction in the absence of an associated inflammatory response. In contrast, necrosis is a pathological mode of cell death that occurs under circumstances of severe cellular injury/trauma. Necrotic cell death involves cell swelling and organelle disruption, followed by lysis and release of cellular debris. This form of cell death may cause damage to surrounding tissue due to the inappropriate triggering of an inflammatory response (2). Apoptosis occurs during normal mammalian development and also plays an important role in subsequent tissue homeostasis by balancing cell division with cell death (3). Apoptosis has also been ascribed a role in several disease states including malignancy and neurodegenerative disorders, in which deregulation or inappropriate activation of the apoptotic program contributes to the observed pathology. The physiological importance of apoptosis in the maintenance of tissue homeostasis, and the observation of apoptotic cell death in age-related degenerative disorders such as Alzheimer’s disease and Parkinson’s disease (reviewed in ref. 4), suggests that the regulation of apoptosis may be a critical parameter for consideration in studies of aging. *These authors have contributed equally to this work. From: Methods in Molecular Medicine, Vol. 38: Aging Methods and Protocols Edited by: Y. A. Barnett and C. R. Barnett © Humana Press Inc., Totowa, NJ

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This chapter outlines some current procedures for the detection of apoptosis and the analysis of intracellular molecular events important in apoptotic pathways. Biochemical events include the generation of reactive oxygen species (ROS) and disruption of mitochondrial transmembrane potential (∆ψm). The methods described in this chapter all utilize a flow cytometer for quantitative analysis of data. Several techniques (e.g., propidium iodide [PI] or terminal deoxyuridine triphosphate [dUTP] nick end labeling [TUNEL]) assay may, however, be adapted for use with a fluorescent microscope. Flow cytometric analysis has the advantage of a rapid assessment of large numbers of cells in a highly quantitative, nonsubjective manner. In addition, flow cytometry enables the parallel assessment of two and possibly three parameters in the same cell. Cell sorting may also be an option for some users (see also subsequent sections).

1.1. Translocation of Phosphatidylserine Several studies have shown that phosphatidylserine (PS) is asymmetrically distributed and is preferentially located in the inner leaflet of the plasma membrane (PM) (5). In the early stages of apoptosis in virtually all cell types, redistribution of membrane phospholipids results in the exposure of PS on the outer membrane (6). PS exposure has been shown to be tightly coupled to other apoptosis-associated changes (7), and seems to be stimulus independent (6). Externalization of PS, following the induction of apoptosis, can be readily detected using recombinant Annexin V, a protein that binds with high affinity to PS in a Ca2+-dependent manner (8). Because necrotic cells have lost their membrane integrity, they may also stain positive with Annexin V. However, dual staining with PI enables membrane-disrupted cells to be readily distinguished. Secondary necrotic cells are apoptotic cells which have subsequently lost membrane integrity (see Subheading 4.1.1.), and therefore cannot be distinguised from primary necrotic cells using this method. As Annexin V detects a cell surface marker, no fixation or permeabilization is required for the procedure. The cells are therefore analyzed “alive” and may be recovered by fluorescence-activated cell sorter (FACS) sorting for further analytical purposes. This method also facilitates dual labeling for surface antigens that recognize native antigen conformations.

1.2. DNA Fragmentation The degradation of DNA into oligonucleosomal sized fragments of 180–200 basepairs by specific endonuclease activity is a major biochemical event during apoptosis in most cell types (9). This DNA fragmentation was originally observed as a ladder pattern using agarose gel electrophoresis. However, this method of detection is essentially qualitative and does not allow for the identification of subpopulations of apoptotic cells. Several flow cytometric methods

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have been described that allow the quantitative analysis of DNA fragmentation as well as the parallel measurement of other parameters such as cell cycle and antigen expression (10,11). The most commonly used of these methods is the TUNEL assay. The TUNEL assay is based on the addition of biotinylated dUTP nucleotide to 3' hydroxyl termini at DNA strand breaks. This reaction is catalyzed by the enzyme terminal deoxynucleotidyl transferase that repetitively adds deoxyribonucleotide to the 3' hydroxyl termini of DNA. Fluoresceinated avidin is then employed in a second step reaction to fluorescently label the DNA strand breaks, thus allowing the detection of DNA strand breaks by flow cytometry.

1.3. Reactive Oxygen Species The generation of ROS and alterations in the cellular redox state have been reported to be a common biochemical event in apoptosis (12). Moreover, ROS have been proposed to be putative second messengers in the initiation of apoptosis. The production of ROS during cytotoxic drug induced apoptosis and inhibition of apoptosis by antioxidants supports this view (12). Oxidative stress is also believed to play a role in Parkinson’s disease and amylotrophic lateral sclerosis disease states, the latter of which has been linked to genetic lesions in a cellular antioxidant pathway (4). Possible sources of intracellular ROS include the depletion of cellular antioxidants such as glutathione, disruption of mitochondrial respiration, and the activation of oxidant-producing enzymes such as NADPH oxidase. The fluorescent probes 2',7'-dichlorofluorescein diacetate (DCFH/DA) and dihydorethidium (DHE) may be used for the measurement of intracellular peroxide and superoxide anion levels, respectively. DCFH/DA is cell permeant and is nonfluorescent until the acetate groups are removed by cellular esterase activity and a peroxide group is subsequently encountered. Hydrolyzed, oxidized DFCH/DA fluoresces at 529 nm (FL-1, log scale; see Subheading 3. and Table 1) (Fig. 1A,B) and is unable to leave the cell, thus allowing the measurement of intracellular peroxides by flow cytometry. DHE is also cell permeant and is oxidised to ethidium by superoxide anion. Once oxidised, ethidium is free to intercalate with DNA in the nucleus whereupon it emits fluorescence at 605 nm (FL-2) (see Fig. 1A,C).

1.4. Mitochondrial Transmembrane Potential Alterations Emerging evidence suggests a central role for mitochondria during apoptosis induced by a diverse range of stimuli in a number of cell types. Indeed, several groups have proposed mitochondria as the central executors of apoptosis (reviewed in ref. 13). Mitochondrial events during apoptosis include release of cytochrome-c and disruption of the transmembrane potential (∆ψm) (13). Dis-

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Table 1 Summary of Assays and Probes Described in this Chaptera Probe/Assay

Parameter measured

Annexin-V TUNEL Antigen analysis PI DHE DCFH/DA JC-1

PS translocation DNA fragmentation Antigen expression DNA content Superoxide anion Peroxide Mitochondrial membrane potential

Emission (nm)

Channel

515 515 580 620 605 529 590

FL-1 FL-1 FL-2 FL-2 FL-2 FL-1 FL-2

aThe

table includes the emission peak of probes and the channel through which data should be collected and analyzed.

ruption of ∆ψm is believed to occur through permeability transition (PT), a process that involves the opening of the mitochondrial PT pore, allowing release of solutes 1.5 kDa and smaller and subsequent disruption of ∆ψm. Importantly, inhibitors of PT also inhibit apoptosis in several models of apoptosis, supporting the view that disruption of mitochondrial function is central to the apoptotic process. The cell permeant fluorescent probe JC-1 can be employed to monitor changes in ∆ψm in intact cells. In the presence of a high ∆ψm JC-1 forms what are termed J-aggregates that fluoresce strongly at 590 nm (FL-2). Reduced ∆ψm results in a reduced FL-2 signal in JC-1-stained cells (Fig. 2A,B). This method has been demonstrated to be quantitative in addition to qualitative and allows subpopulations of cells with different mitochondrial properties to be identified (14). 2. Materials 2.1. Annexin V Assay 1. Fluoresceinated Annexin V (Annexin V-FITC) (e.g., Bender MedSystems, Heidelberg, Germany). Protect from light and store at –20°C. 2. Binding buffer: 10 mM N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES)/NaOH, pH 7.4, 140 mM NaCl, 2.5 mM CaCl2. Store at 4°C. 3. PI. Protect from light and store at 4°C. 4. Phosphate-buffered saline (PBS): 8.06 mM Na2HPO4, 1.47 mM KH2PO4, pH 7.4; 2.27 mM KCl, and 137 mM NaCl.

2.2. Terminal dUTP Nick End Labelling (TUNEL) Assay 1. Fixation buffer: 2% (w/v) p-Formaldehyde in PBS, pH 7.4 (see Subheading 2.1. for PBS formulation).

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Fig. 1. (A) Schematic diagram illustrating the analysis of intracellular peroxides and superoxide using the fluorescent probes DCFH/DA and DHE. Hydrolysis and oxidation of DCFH/DA causes it to fluoresce in FL-1, while the oxidation of DHE to ethidium results in an increase in fluorescence in FL-2 owing to the intercalation of ethidium with cellular DNA. (B) Production of peroxides in DU145 prostate cancer cells treated with camptothecin. Peroxide levels were assessed in untreated DU145 prostate cells (dashed line) treated with 10 µg/mL of camptothecin (solid line) and in cells treated with 1 mM H2O2 (dotted line) for 1 h, as described in Subheading 2.3. After 1 h of treatment with 10 µg/mL of camptothecin, there is an increase in peroxide production that can be seen as a shift to the right in relative fluorescence. (C) Measurment of superoxide anion in retinal cell primary cultures after 24 h. Retinal cells cultures display high levels of apoptosis after 24 h in culture (see Fig. 4-2). Staining cells with DHE reveals a significant increase in superoxide levels at 24 h (solid line) relative to immediately isolated cells (dashed line).

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Fig. 2. (A) Schematic diagram of mitochondrial events during apoptosis. Intact mitochondria display high transmembrane potential (∆ψm) and fluoresce strongly in FL-2 when stained with JC-1. Apoptotic mitochondria undergo permeability transition resulting in a release of solutes 1.5 KDa and smaller and a loss transmembrane potential, consequently displaying reduced fluorescence in FL-2 when stained with JC-1. (B) Alterations in ∆ψm in NSO myeloma cells upon camptothecin treatment. NSO myeloma cells were treated with 30 mM ammonia for 18 h. Depolarization in ∆ψm was assessed as described in Subheading 2.3., using JC-1. ∆ψm depolarization can be monitored by measuring the fluorescence in FL-1. Membrane depolarization results in an increase in FL-1 flourescence. Treated cells (solid line) show an increase in FL-1 fluorescence relative to untreated cells (dashed line).

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2. Elongation buffer: 0.2 M Potassium cacodylate; 25 mM Tris-HCl, pH 6.6; 2.5 mM cobalt chloride; 0.25 mg/mL of bovine serum albumin (BSA); 100 U/mL of terminal deoxynucleotidyl transferase (TdT) (e.g., Boehringer Mannheim, East Sussex, UK); 0.5 nM biotin-16-dUTP (e.g., Boehringer Mannheim). Make fresh as required. 3. Staining buffer: Dilute 20× saline citrate buffer (0.3 M sodium citrate; 3 M NaCl, pH 7.0) to 4×; add 2.5 mg/mL of fluoresceinated avidin, 0.1% (v/v) Triton X100; 5% (w/v) nonfat dried milk, to give 0.6 M NaCl and 0.06 M sodium citrate. Staining buffer is freshly made and protected from light.

2.3. Detection of Intracellular ROS 1. DCFH/DA (Molecular Probes, Leiden, The Netherlands) prepared as a 5 mM stock in dimethyl sulfoxide (DMSO). Protect from light and store at –20°C. 2. DHE (Molecular Probes) prepared as a 10 mM stock in DMSO. Protect from light and store at –20°C.

2.4. Measurement of Mitochondrial Transmembrane Potential 1. JC-1 (Molecular Probes), made as a 5 mg/mL stock in DMSO. Protect from light and store at –20°C.

3. Methods

3.1. Annexin V Assay 1. Harvest 1–2.5 × 105 cells and resuspend in 200 µL of binding buffer. 2. Dilute Annexin V–flourescein isothiocyanate (FITC) as recommended by the manufacturer. 3. Add diluted Annexin V–FITC and incubate for 10 min at room temperature, in the dark. 4. After a wash in PBS, resuspend cells in binding buffer and add PI (see Subheading 4.1.2.) (to a final concentration of 5 µg/mL). 5. Quantitate Annexin V binding and PI staining by flow cytometry (FL-1 and FL-2 respectively) (see Note 3) (see Subheading 3. and Table 1) (Fig.3).

3.2. TUNEL Assay 1. Fixation and permeabilization (see Note 4): Harvest approx 1 × 106 cells and suspend in 1 mL PBS. Add 1 mL of 2% (w/v) p-formaldehyde fixation buffer (see Note 5). Leave on ice for 15 min. Wash once in PBS and resuspend in 2 mL of –20°C 70% (v/v) ethanol. Leave at –20°C for at least two hours or up to two weeks. 2. Elongation: Wash fixed/permeabilized cells twice in PBS and resuspend in 50 µL of elongation buffer. Incubate at 37°C for 30 min. Wash twice in PBS. 3. Staining: Resuspend cells in 100 µL of staining buffer and incubate in the dark at room temperature for 30 min.

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Fig. 3. Annexin V/PI dual staining of human T cells treated with Actinomycin D. Jurkat cells were treated for 4 h with 15 mg/mL of Actinomycin D. Cells were stained with Annexin V-FITC (FL-1, y-axis) and PI (FL-2, x-axis) as described in Subheading 2.1. Normal cells are negative for both Annexin and PI and appear in the lower left quadrant. Apoptotic cells with intact membranes stain with Annexin, but not with PI and therefore appear in the upper left quadrant. Primary and secondary necrotic cells have disrupted membranes and stain with both Annexin V and PI (upper right quadrant). 4. Wash twice in PBS and keep on ice and in the dark until read on a flow cytometer. 5. Measure green fluorescence (labeled DNA strand breaks) following excitation at 488 nm using a 525 nm band pass filter (FL-1, log scale; see Subheading 3. and Table 1).

3.2.1. Analysis of Cell Cycle in Conjunction with TUNEL Assay After the TUNEL assay procedure has been completed resuspend cells in 1 mL of PBS containing 5 µg/mL of PI, and 0.1% DNase-free RNase A. Cell cycle analysis can then be carried by measuring the red fluorescence of PI at >600 nm (FL-2, linear scale; see Subheading 3. and Table 1) (Fig. 4-1). 3.2.2. Analysis of Antigen Expression in Conjunction with TUNEL Assay The combination of TUNEL staining with immunofluorescence labeling of a cell-specific antigen allows the subsequent analysis of apoptosis in specific subpopulations. Cells may be labeled for antigen expression prior to or following fixation/permeabilization depending on the nature of the epitope. It is recommended that it first be determined whether epitope labeling is affected by the fixation/permeabilization process. The following procedure detects an intra-

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Fig. 4-1. Analysis of apoptosis by TUNEL and cell cycle in human leukemic cells treated with Etoposide. HL-60 cells (myeloid leukemia cell line) were treated for 3 and 6 h with 40 µM Etoposide (VP16). Cells were stained with FITC using the TUNEL procedure as described in Subheading 2.2. Dual staining with PI enables cell cycle parameters to be assessed in parallel with apoptosis. This data shows that after a 3-h incubation with VP16 the majority of S-phase cells have initiated endonucleolytic DNA cleavage. Some cleavage is also evident in G0/G1 cells, whereas G2/M cells are relatively resistant. After 6 h, most of the unlabeled G2/M cells have disappeared. Some of these cells may have labeled with FITC, although most appear to have cycled through to G0/G1, where the cell cycle is blocked.

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Fig. 4-2. The detection of rod photoreceptor apoptosis in a primary retinal cell culture using dual labeling for rhodopsin expression (rod specific protein) and TUNEL. (A) Retinal culture stained with anti-rhodopsin antibody that was subsequently labeled with a phycoerytherin-conjugated secondary antibody. The encircled rhodopsin-positive population (high FL-2, y-axis) can be readily distinguished from nonrod cells in the retinal culture. (B) TUNEL of rhodopsin-positive cells after 24 h in culture displaying high levels of apoptosis. The dashed line represents a negative control of cells stained without the TdT enzyme while the solid line represents TUNEL stained rhodopsinpositive cells.

cellular antigen. The cells must therefore be labeled with the specific antibody following fixation. 1. Once the TUNEL assay protocol is completed, incubate cells in appropriately diluted (using PBS/1% BSA, w/v) primary antibody for 60 min. 2. Wash twice in PBS and once in PBS/BSA (0.1%, w/v). 3. Incubate cells in appropriately diluted (using PBS/1% BSA) phycoerythrinconjugated secondary antibody for 30 min.

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4. Antigen expression is analyzed by measuring red fluorescence at >600 nm (FL-2, log scale; see Subheading 3. and Table 1) (Fig. 4-2).

3.3. Measurement of ROS 3.3.1. Measurement of Intracellular Peroxide Levels 1. Incubate cells (see Notes 6 and 7) (5 × 105/mL) with 5 µM DCFH/DA, for 1 h at 37°C, in the dark. 2. Assess peroxide levels using a FACScan flow cytometer with excitation and emission settings of 488 nm and 530 nm, respectively (FL-1, log scale; see Subheading 3. and Table 1) (Fig. 1B).

3.3.2. Measurement of Intracellular Superoxide Levels 1. Incubate cells (see Subheadings 4.3.1. and 4.3.2.) (5 × 105/mL) with 10 µM hydroethidine, for 15 min at 37°C, in the dark. 2. Superoxide levels are assessed using a FACScan flow cytometer with excitation and emission settings of 488 nm and 600 nm respectively (FL-2) (see Subheading 3. and Table 1; Fig. 1C).

3.4. Measurement of Mitochondrial Transmembrane Potential 1. Incubate cells (5 × 105/mL) with 5 µg/mL of JC-1 for 15 min at 37°C, in the dark. 2. Ratiometric measurements are performed using a FACScan flow cytometer, in FL-1 and FL-2 (log scales; see Subheading 3. and Table 1) (Fig. 2A).

3.5. Acquisition and Analysis of Flow Cytometric Data The assays described in this chapter were performed on a FACScan flow cytometer (Beckton Dickinson). An excitation source of 488 nm was obtained using a 15-mW air-cooled argon ion laser. Fluorescence emission was collected through a 530/30 band pass filter (FL-1) on a log scale for TUNEL, DCFH/DA, JC-1, and Annexin-V assays and a 585/42 band pass filter log scale (FL-2) for PI, DHE, JC-1, and antigen labeling assays, while cell cycle analysis (PI) was conducted using a linear scale (FL-2 area). A minimum of 5000 events were collected for each sample. CellQuest™ software version 1.1.1 was used for both acquisition and analysis of data. 4. Notes

4.1. Annexin V Assay 1. Under physiological conditions apoptotic cells are phagocytosed prior to loss of membrane integrity. 2. Following addition of PI, cells should be analyzed with minimal delay, as PI may eventually leak into normal and apoptotic cells. 3. As some cells stain very brightly with FITC, it may be necessary to use FACS compensation (FL-2-FL-1) during data acquisition (see Subheading 3. and Table 1).

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4.2. TUNEL Assay 4. If an assessment of necrosis is required in a given population this can be achieved prior to fixation by employing the PI exclusion assay as described in Subheading 2.1. 5. This avoids any clumping of cells that may occur if a 1% (w/v) p-formaldehyde fixation buffer were added directly to samples.

4.3. Detection of Reactive Oxygen Species 6. Cells can be treated with apoptosis-inducing agents either before, after, or during the incubation period, depending on the time point at which ROS levels are to be measured. 7. As a positive control for peroxide production, cells may be treated with 1 mM H2O2 for 30–60 min (see Fig. 1B).

Acknowledgments This work was supported by the Foundation for Science and Technology (Fundação para a Ciência e a Tecnologia), Lisbon, Portugal, RP Ireland Fighting Blindness, and the EU Biotech Programme. References 1. Kerr, J. F. R., Wyllie, A. H., and Currie, A. R. (1972) Apoptosis, a basic biological phenomenon with wider implications in tissue kinetics. Br. J. Cancer 26, 239–245. 2. Trump, B. F., Beresky, I. K., and Osornio-Vargas, A. R., eds. (1981) in Cell Death in Biology and Pathology, Chapman and Hall, New York. 3. Raff, M. C. (1992) Social controls on cell survival and cell death. Nature 365, 397–400. 4. Gorman, A. M., McGowan, A. J., O’Neill, C., and Cotter, T. G. (1996) Oxidative Stress and apoptosis in neurodegeneration. J. Neurol. Sci. 139, 45–52. 5. Devaux, P. F. (1991) Static and dynamic lipid asymmetry in cell membranes. Biochemistry 30, 1163–1173. 6. Martin, S. J., Reutelingsperger, C. P. M., and Green, D. R. (1996) Annexin V- a specific probe for the detection of phosphatidylserine exposure on the outer plasma membrane leaflet during apoptosis, in Techniques in Apoptosis: A Users Guide (Cotter, T. G. and Martin S. M., eds.), Portland Press, London, pp. 107–119. 7. Martin, S. J., Reutelingsperger, C. P. M., McGahon, A. J., Rader, J., van Schie, R. C. A. A., La Face, D. M., and Green, D. R. (1995) Early redistribution of plasma membrane phosphatidylserine is a general feature of apoptosis regardless of the initiating stimulus: Inhibition by overexpression of Bcl-2 and Abl. J. Exp. Med. 182, 1545–1556. 8. Swairjo, M. A., Concha, N. O., Kaetzel, M. A., Dedman, J. R., and Seaton, B. A. (1995) Ca2+-bridging mechanism and phospholipid head group recognition in the membrane-binding protein Annexin V. Nat. Struct. Biol. 2, 968–974. 9. Arends, M. J., Morris, R. G., and Wyllie, A. H. (1990) Apoptosis: the role of the endonuclease. Am. J. Pathol. 136, 593–608.

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10. Cotter, T. G. and Martin, S. J., eds. (1996) in Techniques in Apoptosis, Portland Press, London. 11. Carmody, R. J., McGowan, A. J., and Cotter, T. G. (1998) Rapid detection of rod photoreceptor apoptosis by flow cytometry. Cytometry, 33, 89–92. 12. McGowan, A. J., Ruiz-Ruiz, M. C., Gorman, A. M., Lopez Rivaz, A., and Cotter, T. G. (1996) Reactive oxygen intermediates (ROI): common mediators of poly(ADPRibose)polymerase (PARP) cleavage and apoptosis. FEBS Lett. 392, 299–303. 13. Kroemer, G. (1997) Mitochondrial implication in apoptosis. Towards an endosymbiont hypothesis of apoptosis in evolution. Cell Death Different. 4, 443–456. 14. Salvioli, S., Ardizzoni, A., Franceschi, C., and Cossarizza, A. (1997) JC-1, but not DiOC6(3) or rhodamine 123, is a reliable fluorescent probe to assess ∆ψm change in intact cells: Implications for studies on mitochondrial functionality during apoptosis. FEBS Lett. 411, 77–82.

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6 Raf-1 Protein Kinase Activity in T Cells from Aged Mice Christopher J. Kirk and Richard A. Miller 1. Introduction Most of our models of signal transduction through the T-cell receptor (TCR) involve components and pathways first described in T-cell clones and T-cell lymphomas such as the Jurkat cell line (1). These studies, while providing valuable insights, are not always reliable guides to the analogous biochemical events in cells freshly isolated from live donors (2). Thus, studies of the effects of aging on T-cell activation must frequently begin with a detailed study of the responses of cells from young individuals. Studies of aging effects involve additional challenges, including the difficulty of purifying sufficient numbers of cells from specific subsets, and allowing for the inherent variability among donors of any age. The mitogen-activated protein kinase (MAPK) pathway involves the sequential activation of three kinases—Raf-1, mitogen-activated protein kinase (MEK), and extracellular-signal-regulated kinase (ERK)—and plays an important role in T-cell activation (3). Here we describe an in vitro kinase assay for Raf-1, which utilizes Raf-1 specific antibodies and a recombinant substrate, to assess age-related differences in Raf-1 activation in mouse splenic CD4+ T-cells stimulated through the TCR. The problems involved in analyzing Raf-1 activity levels in freshly isolated T-cells are similar to those that are likely to be faced in the study of age-related alterations in the signal transduction of other cell types.

1.1. Isolation of Primary Lymphocytes T-cell populations are made up of many distinct subsets, each with different activation requirements, some of which change systematically with age. Thus, From: Methods in Molecular Medicine, Vol. 38: Aging Methods and Protocols Edited by: Y. A. Barnett and C. R. Barnett © Humana Press Inc., Totowa, NJ

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experiments on unseparated T-cell pools are likely to confound age effects on activation pathways with the effects of subset transitions. Aging leads to an increase in memory cells in both the CD4 and CD8 pools as measured by the increase in CD44hi cells (4). The naïve and memory pool can be further subdivided based on differences in expression of the membrane glycoprotein, P-glycoprotein, which also shows increased expression with age (5). It thus seems reasonable to study activation pathways first in separated CD4 and CD8 subsets, and to include studies of separated subsets wherever practical. Purification of these subsets from spleens begins with the depletion of erythrocytes and B cells by differential centrifugation and panning on anti-IgGcoated plates, respectively (6). These procedures typically produce 25–35 × 106 T-cells from a single mouse spleen, among which 85–95% express the CD3 ε-chain characteristic of T-cells. For CD4+ T-cell purification, as is described in this chapter, CD8 cells are depleted by incubation with anti-CD8 antibodies followed by removal with anti-IgG coated magnetic beads. Multiple subsets can be depleted simultaneously; for example, addition of anti-CD44 and antiCD8 can be used to purify CD4 naïve cells (i.e., CD4+CD45RBhi). Yields of 15–20 × 106 CD4+ T-cells are usually obtained from a single spleen. The numbers of memory and naïve cells that can be obtained from a single spleen vary depending on the age of the mouse (see previous paragraph); experiments involving cell types present only at low frequencies, such as memory cells from young mice, may require pooling spleens of mice of the same age in individual experiments.

1.2. Stimulation of T-Cells with Monoclonal Antibodies to Cell Determinants Because the number of cells obtained from a single spleen is on the order of 5–20 × 106 depending on the subset isolated, it is necessary to develop experimental conditions that permit the stimulation and assay of samples as small as 2–5 × 106 cells. Therefore, choosing appropriate stimuli as well as optimizing stimulation conditions will influence the level of activation of the target enzyme and are important factors in developing reliable assays for aging studies. Activation of T-cells can be achieved through the use of lectins, such as concanvalin A or phytohemagglutinin, that bind to unknown cell surface determinants. T-cells can also be stimulated using monoclonal antibodies specific for components of the TCR and for other cell surface markers (e.g., CD4 or CD28), or stimulated using pharmacological agents such as phorbol esters and calcium ionophores. We describe here the use of monoclonal antibodies to the ε-chain of the TCR/CD3 complex and CD4 to stimulate CD4+ T-cells isolated from young and old mice. We feel that this provides a physiologically relevant stimulation for a polyclonal population of T-cells. Studies using T-cells from transgenic

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mice that express a rearranged antigen-specific TCR will, in future work, provide analogous information about responses to peptides presented by accessory cells, which may more closely mimic the in vivo activation of T-cells. The T-cells are incubated on ice with the stimulating antibodies followed by crosslinking at 37°C with anti-rat IgG to initiate activation. We use purified ascites fluid as our source of stimulating antibodies, although commercially available monoclonals can also be used. We have found that anti-rat IgG can effectively crosslink anti-CD3ε, even when using hamster clone 145–2C11 as the anti-CD3 reagent. Furthermore, anti-rat IgG has the advantage of being able to crosslink the rat antibodies used as costimulators (such as anti-CD4). The phorbol ester phorbol myristate acetate (PMA) (10 ng/mL) activates Raf-1 in mouse T-cells, and can be used as a positive control. The commonly used human lymphoma cell line Jurkat will also activate Raf-1 in response to either PMA or to anti-CD3 antibodies (7), although it should be noted that activation of human peripheral blood lymphocytes (PBL) or Jurkat cells with anti-CD3ε does not require the addition of a crosslinking reagent such as anti-rat IgG. Antibodies to other cell surface markers have been shown either to augment TCR-driven T-cell activation or to activate signaling pathways independently (8,9). In the case of Raf-1 we have evidence that the costimulatory molecule CD28 plays a role in Raf-1 activation of CD4+ T-cells (21) while others have shown that CD38 ligation can activate Raf-1 in Jurkat (10). Little is known about whether aging affects signal transduction from these other cell surface receptors, although our own data suggest that Raf-1 induction by anti-CD28 antibodies in CD4 cells is unaffected by aging in mice. When stimulating T-cells with antibodies to the TCR or other surface molecules, it is important to determine how time of stimulation and dose of antibodies affect activation of the target enzyme. We found that in splenic CD4+ T-cells, Raf-1 activation occurs within 1 min of crosslinking anti-CD3ε and anti-CD4 (Fig. 1). Initial descriptive studies of aging effects should include data on the kinetics of activation in both old and young samples, to see if apparent differences in the level of activation represent merely an alteration in the time course for activation. Furthermore, concentration curves of the antibody or antibodies used must be determined to find the optimal stimulation conditions. We titrated the amounts of anti-CD3ε and anti-CD4, which were purified from ascites fluid, to determine the optimal stimulation conditions for Raf-1 activation in CD4+ T-cells. We found that anti-CD3 concentrations between 0.2 and 7 µg/mL could effectively stimulate Raf-1 in CD4+ T-cells. In our hands, addition of anti-CD4 at concentrations between 2 and 8 µg/mL augments stimulation by anti-CD3, while anti-CD4 costimulation with amounts above or below this range inhibits Raf-1 activation by anti-CD3. Furthermore, anti-CD4 antibodies by themselves could not stimulate Raf-1 (21).

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Fig. 1. (A) Analyzing Raf-1 kinase activity in CD4+ T cells from young, middleaged, and old mice. Raf-1 was immunoprecipitated from 3 × 106 CD4+ T cells that were stimulated with crosslinked anti-CD3+ anti-CD4 for the indicated times or crosslinked anti-DNP for 2 min (lanes marked C) and incubated with a KIMEK substrate and [32P]ATP in an in vitro kinase assay. Reaction products were resolved by 10% SDS-PAGE and visualized by a PhosphorImager. Arrow indicates the 50-kDa migrating band of KIMEK. (B) Data from several experiments (N = 5 for 1 and 2 min and N = 4 for 5 and 10 min) are presented as -fold increase in Raf-1 kinase activity over anti-DNP-stimulated samples. *p < 0.05 for Young vs Old and Young vs Middleaged by one-way ANOVA followed by a Student–Newman–Keul’s post hoc test.

1.3. Measurement of Raf-1 Kinase Activity Raf-1 is a serine/threonine kinase of 74 kDa that is activated by association with activated p21Ras. Besides its association with Ras, Raf-1 activation also

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requires either tyrosine or serine/threonine phosphorylation (11). The importance of Raf-1 activity in T-cell activation and interleukin-2 (IL-2) production has been shown using Jurkat cells transfected with dominant negative forms of Raf-1. In this system, inhibition of Raf-1 blocks IL-2 production through the TCR (12,13). It has also been shown in Jurkat cells that Raf-1 activation through the TCR involves a protein kinase C dependent step that may be unique to T-cells (14). As the first kinase activated in the MAPK pathway, Raf-1 phosphorylates and activates the dual specificity kinases MEK1/2, which, in turn, activate ERK1/2 (11). Early studies of Raf-1 in vitro kinase activity utilized either nonspecific substrates, such as histone H1, or Raf-1 autophosphorylation (11,15). Other measures of Raf-1 activity involved analyzing the retardation in mobility of Raf-1 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDSPAGE), presumably due to activating phosphorylation. However, these assays lack the specificity of analyzing in vitro phosphorylation of a substrate known to be phosphorylated by Raf-1 in vivo. Furthermore, as Raf-1 contains multiple sites of phosphorylation, including those that inhibit kinase function, the slowly migrating form of Raf-1 in SDS-PAGE may contain a collection of differentially phosphorylated species of varying levels of activity (11). With the cloning of MEK into plasmids that allow for protein production and purification from E. coli, a suitable substrate for in vitro Raf-1 kinase assays is available (16). We used a plasmid encoding a MEK construct that contains a lysine to methionine substitution at residue 97 in the ATP binding site, and therefore lacks kinase activity (16). This kinase-inactivated MEK, which we call KIMEK, has been used as a substrate to determine the enzymatic activity of Raf-1 in several cell systems, including Jurkat (7). In our hands, plasmid-derived KIMEK contains multiple species that migrate closely to the predicted molecular weight for KIMEK of 50 kDa (Fig. 1), each of which is phosphorylated by Raf-1 in in vitro kinase assays. These bands are likely to represent degradation products of the KIMEK, and have been noted in other publications using this substrate (16,17). Another version of KIMEK is commercially available as a GST fusion protein from Upstate Biotechnology (Lake Placid, NY, USA). Raf-1 function is more difficult to quantify in freshly isolated lymphocytes than in cell lines, in part because Raf-1 protein levels are lower in primary cell isolates. T-cells isolated from spleens are in a quiescent state and contain a small cytosolic compartment as compared to a T-cell clone or Jurkat cell, which are the size of T-cell blasts. In fact, Raf-1 protein levels are approximately tenfold lower per cell in quiescent T-cells than in T-cell blasts (18), and we have found that Raf-1 protein expression in Jurkat is approximately fivefold greater than in splenic CD4+ T-cells (Kirk and Miller, unpublished results).

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Furthermore, limitations in the number of T-cells that can be obtained from each donor make it critical to develop and validate methods to measure kinase activity from the lysates of 2–5 × 106 cells. The in vitro kinase assay for Raf-1 involves the immunoprecipitation of Raf-1 from cell lysates followed by incubation with the KIMEK substrate and [32P]ATP. Several precautions in Raf-1 isolation and kinase measurement help to ensure high levels of kinase activity, low levels of background phosphorylation, and maximal difference between age groups. Anti-Raf-1 antibodies can currently be purchased in either mouse monoclonal or rabbit polyclonal forms. Although we have achieved the best results with rabbit polyclonal sera, other forms should be tested in different systems (e.g., using human cells, or different strains of mice or cell types). Increasing the stringency of a lysis buffer will reduce coprecipitation of other kinases, which might otherwise increase the background phosphorylation of bands other than KIMEK. Background phosphorylation can also be reduced by preclearing the lysates with Protein G–Sepharose prior to immunoprecipitation. 2. Materials 1. Mice. We use specific-pathogen free male (BALB/c × C57BL/6)F1 mice purchased from the National Institute on Aging contract colonies at the Charles River Laboratories (Kingston, NJ, USA). Initial work should use animals of several different ages, avoiding the use of animals so old that they are likely to be ill. In our own studies we typically use mice aged approx 3–6 mo, 12–14 mo, and 18–22 mo from strains in which 50% mortality is not reached until 24–26 mo of age. Mice over the age of 15 mo should be examined at necropsy and those with skin lesions, splenomegaly, or macroscopically visible tumors should not be used. 2. Hanks balanced salt solution is supplemented with 0.2% bovine serum albumin (BSA) (HBSS–BSA). It should be used and stored at 4°C unless otherwise noted. 3. Lympholyte-M™ (Cedarlane Laboratory, Ontario, Canada). 4. Antibodies: Antibodies to CD3ε (clone 145-2C11), the dintrophenyl hapten (DNP) (clone UC8), CD4 (clone GK1.5), and CD8 (clone 53.6-7) were produced in our laboratory from cell lines purchased from the American Type Culture Collection (Rockville, MD, USA). Anti-CD62L (clone Mel-14) can be purchased from Pharmingen (San Diego, CA, USA). Anti-rat IgG used in cell stimulation was purchased from Sigma (St. Louis, MO, USA). We find that most lots of goat anti-rat IgG are suitable for crosslinking hamster antibodies (such as anti-CD3ε), and are often superior to anti-hamster antibodies in this system; anti-rat Ig secondary antibodies also have the advantage that they provide crosslinking between hamster anti-CD3 and rat anti-CD4 or anti-CD8 antibodies. Magnetic beads conjugated to anti-rat IgG are obtained from PerSeptive Diagnostics, Cambridge, MA, USA, and polyclonal anti-Raf-1 from Santa Cruz Biotech, Santa Cruz, CA, USA. 5. Preparation of anti-Ig-coated dishes for B-cell depletion. Plates of 150 mm diameter are incubated with 35 mL of phosphate-buffered saline (PBS) containing

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9. 10.

12. 13.

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1 µg/mL of rabbit anti-mouse IgG (H + L) overnight at 4°C on a level surface. Plates can be stored for 4–7 d at 4°C or kept for long-term storage at –20°C. Prior to use the plates are washed 3× with PBS to remove unbound antibody and incubated for at least 1 h with 30 mL of HBSS–BSA at 4°C. Petri dishes of 100 mm diameter can be prepared in the same way using one-third the volume of rabbit anti-mouse IgG. RPMI supplemented with 10% FCS (RPMI–FCS). Protein G–Sepharose (Pharmacia, Piscataway, NJ, USA) RIPA buffer: 10 mM sodium phosphate, pH 7.0; 150 mM NaCl, 2 mM EDTA, 1% sodium deoxycholate, 1% Nonidet P-40 (NP-40), 0.1% sodium dodecyl sulfate (SDS); 0.1% 2-mercaptoethanol, 50 mM NaF, 200 µM Na3VO4, 2 µg/mL of aprotinin; 0.5 µg/mL of leupeptin; and 2 µg/mL of pepstatin. Add 0.1 mM of phenylmethyl sulfonyl flourite (PMSF) just prior to use. PAN buffer: 10 mM PIPES, pH 7.0, 100 mM NaCl, 20 µg/mL of aprotinin. Recombinant kinase inactive MEK substrate (KIMEK). We used a plasmid encoding a KIMEK containing a 6 histidine tag (the generous gift of Gary L. Johnson, National Jewish Center for Immunology, Denver, CO, USA), that was expressed and purified as described (19). Commercially available KIMEK in the form of a glutathione-S-transferase (GST) fusion protein is available from Upstate Biotechnology (Lake Placid, NY, USA, cat. no. 14–159). Kinase buffer: 20 mM PIPES, pH 7.0, 75 mM NaCl, 10 mM MnCl2, 20 µg/mL of aprotinin. Nitrocellulose (Schleicher and Shuell, Keene, NH, USA).

3. Methods 3.1. Purification of Splenic CD4+ T-Lymphocytes 3.1.1. T-lymphocyte Purification 1. Kill mice by cervical dislocation or CO2 asphyxiation. Experiments should be age balanced, with mice of different age groups killed in each experiment. 2. Wet down fur with 70% ethanol and remove spleens to separate Petri dishes (6 mm diameter) containing 6 mL of HBSS–BSA at room temperature. 3. Rub spleens between frosted glass slides to obtain a single cell suspension. 4. Pass suspension through a sheet of nylon mesh to remove cell clumps and connective tissue. Wash plate once with 6 mL of HBSS–BSA and pass through the nylon mesh sheet. Each splenic suspension should be in a volume of 12 mL. 5. Centrifuge splenocytes at ~20°C for 5 min at 500g. 6. To remove erythrocytes, resuspend splenocytes in 8 mL of HBSS–BSA warmed to 37°C. Carefully overlay splenocyte suspension on 4 mL of Lympholyte-M™ that has also been warmed to 37°C. Four milliliters of Lympholyte-M™ has the capacity for 100 × 106 lymphocytes (the approximate number of lymphocytes in a single spleen). Centrifuge at room temperature for 15 min at 600g with the brake off. 7. Remove buffy coat containing lymphocytes and wash with 15 mL of HBSS– BSA. Centrifuge at 4°C for 5 min at 500g. If the Lympholyte-M™ has not been

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sufficiently diluted there will be incomplete cell pelleting. Dilute further in HBSS–BSA if supernatant shows any turbidity following centrifugation. 8. Resuspend lymphocytes in 15 mL of ice-cold HBSS–BSA and add to anti-Igcoated dishes. Incubate at 4°C on a level surface for 40 min. Swirl plates once at the end of the first 20 min to redistribute the cells. Dishes of 150 mm diameter should receive approx 120–150 × 106 erythrocyte-depleted spleen cells; using more cells per dish diminishes cell purity, while using fewer cells diminishes cell yield. 9. Gently separate nonadherent from adherent cells by swirling the plate vigorously and then slowly removing the cell suspension from the tilted dish. It is particularly helpful to remove the last 0.5–1 mL very slowly, allowing the meniscus to collect at the low point of the dish. Wash plate once with 5 mL of ice-cold HBSS– BSA to improve cell yield, again taking care to remove the suspension slowly. Centrifuge cells at 4°C for 5 min at 500g. 10. Resuspend cells in ice-cold HBSS–BSA and count. From 25 to 30 × 106 T-cells can be obtained from a single spleen. Remove an aliquot of cells to perform flow cytometry with an antibody to mouse CD3ε to determine the proportion of T-cells in the resulting population (typically 85–90%).

3.1.2. Negative selection of T cell subsets 1. Dilute T-cells, prepared as described in Subheading 3.1.1., into HBSS–BSA to a concentration of approx 5 × 106 cells/mL. 2. Add antibodies to cell markers of subsets to be depleted (i.e., for CD4+ T-cell purification add antibodies to CD8). The amount of antibody needed is based on trial experiments in which CD8 cell contamination of resulting cells is determined by flow cytometric analysis. In our experiments, we use 1 µg of antibody for 107 cells at a concentration of 5 × 106 cells/mL. 3. Incubate on ice for 20 min with gentle shaking at 5- to 10-min intervals. 4. Prepare anti-rat IgG-coated beads according to the manufacturer’s specifications. Prepare enough for a bead-to-cell ratio of 50:1. Resuspend beads in 0.4 mL of ice-cold HBSS–BSA. 5. Centrifuge cells at 4°C for 5 min at 500g. Wash cells once in 10 mL of cold HBSS–BSA. 6. Resuspend cells in 1 mL of cold HBSS–BSA and add 0.4 mL of bead solution. 7. Incubate on ice for 20 min with shaking every 5 min to ensure suspension of beads. 8. Fill tube to 5 mL with cold HBSS–BSA and pass under magnetic field (PerSeptive Diagnostics, Cambridge, MA, USA), which is kept at 4°C. 9. Carefully remove cell suspension without disturbing the beads aligned along the side of tube. 10. Repeat steps 8 and 9 two to three more times to ensure complete removal of beads. 11. Centrifuge cells at 4°C for 5 min at 500g. Resuspend in cold RPMI supplemented with 10% fetal calf serum (FCS) (RPMI–FCS) and count. An aliquot of ~0.5 × 106 cells should be removed at this stage for flow cytometric analysis to assess the purity of selected cells.

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3.2. T-lymphocyte Activation 1. Aliquot equal numbers of cells into 1.5-mL tubes in a total volume of 1 mL. A sample from each mouse should be saved for analysis of Raf-1 protein expression levels. In our experience we get usable results from samples as small as 2 × 106 cells/sample, although samples of 5 × 106 cells/sample give stronger signals and are thus preferable when cells are not limiting. 2. Incubate cells with antibodies to the TCR/CD4 complex (anti-CD3ε and anti-CD4) or other cell surface determinants. Some aliquots should also be incubated, as a control, with a nonstimulatory antibody such as monoclonal anti-DNP of the same species and isotype as the antibodies used for cell activation. Incubate the samples on ice for 30 min. Invert the tubes several times during the incubation period. 3. Prepare solutions of RPMI–FCS with 5 µg/mL of anti-rat IgG (crosslinking reagent) and RPMI–FCS with PMA (10 ng/mL). Warm to to 37°C. 4. Centrifuge cells at 1000g for 30 s to pellet. 5. Resuspend in 1 mL of warm anti-rat Ig solution or PMA and incubate at 37°C for the desired interval. Raf-1 activation in mouse CD4 T-cells stimulated by antiCD3 and anti-CD4 is detectable within 1 min and typically reaches a plateau level within 5–10 min before subsiding to lower levels over the next half hour. Stimulation with PMA for 5 min serves as a positive control for Raf-1 activation. 6. Stop reaction by centrifugation at 10,000g for 10 s. 7. Wash cells in ice-cold PBS. Centrifuge cells at 10,000g for 10 s. 8. Lyse cells in RIPA buffer, using 0.8 mL for immunoprecipitation or 40 µL for protein expression levels. 9. Incubate in RIPA for 15–30 min. Centrifuge cells at 4°C for 10 min at 13,000g. 10. Remove pellet or transfer supernatants to a new tube.

3.3. In Vitro Kinase Assay of Raf-1 3.3.1. Raf-1 Immunoprecipitation 1. Preclear samples with 20 µL of Protein G–Sepharose (50% slurry) for 0.5–1 h on a rocker at 4°C. Wash the Protein G–Sepharose several times with lysis buffer before use. Pellet beads and transfer supernatants to new tubes. 2. Add anti-Raf-1 antibodies at 1:100 dilution (i.e., 8 µL for 0.8 mL of lysate) and 25 µL of Protein G–Sepharose. Incubate overnight on a rocker at 4°C. 3. Wash samples twice with ice-cold RIPA using centrifugation at 10,000g for 10 s. 4. Wash twice with ice-cold PAN buffer containing 1% NP-40 and once in ice-cold PAN buffer.

3.3.2. Raf-1 Kinase Assay 1. For each sample add 50–100 ng of purified KIMEK and 30 µCi of [γ-32]P-ATP to 30 µL of kinase buffer. The kinase buffer should be prepared for all samples in the same tube to ensure equal loading of KIMEK and [32P]ATP to each sample. 2. Add 30 µL to each sample and incubate for 15 min at 30°C. 3. Stop reaction by addition of 15 µL of 4× reducing sample buffer and boiling.

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4. Run samples on 10% SDS-PAGE and transfer to nitrocellulose. Expose blot to X-ray film or an intensifier screen for phosphor storage image analysis. 5. Densitometric values are corrected for background levels and can be expressed as fold activation by expression of the ratio of the densitometric volumes in the stimulated samples to the anti-DNP-stimulated sample. 6. In a separate 10% SDS-PAGE gel transferred to nitrocellulose, the whole cell lysates should be analyzed by Western blot analysis using the anti-Raf-1 antibodies according to the manufacturer’s instructions.

4. Notes 1. Because of limitations in the yield of T-cell subsets from mouse spleens, it is important to minimize cell loss during the purification procedures. To that end, careful notice should be paid to the overlay of splenocytes on Lympholyte-M™ and the complete removal of the buffy coat following centrifugation. It is also important to ensure complete removal of nonadherent cells from the T plates. The plates can placed under a microscope to determine if nonadherent cells remain. Further washes of the anti-Ig-coated dishes may be helpful. Lastly, it is important to be sure cells are in a single cell suspension, that is, not clumped, prior to the addition of the anti-rat-Ig-coated magnetic beads to minimize cell loss at that step. 2. The level of Raf-1 kinase activation by antibodies to cell determinants is dependent upon antibody dose and time of stimulation. Dose curves for stimulating antibodies will be necessary, as ascites purification and antibody storage will give varying levels stimulating efficacy. 3. The stringency (i.e., detergent type) of the lysis buffer will determine the amount of nonspecific phosphorylation of bands other than the KIMEK substrate. We use RIPA because it has been shown to bring down Raf-1 in the absence of associated proteins such as 14-3-3 (20). Because we find the purity of our KIMEK is 500 basepairs can be amplified. The probes are analyzed by horizontal gel electrophoresis and are detected by silver staining with a nonradioactive method. They also can be stored at –20°C.

3.4. Discontinuous Acrylamide DNA Electrophoresis For DNA separation, CleanGels and the DELECT-buffer Kit can be used. This system allows a size separation of the DNA in a native, nondenaturing manner. The electrophoresis is done in a horizontal manner in the Multiphor II chamber from Pharmacia. This is an easy and useful method to analyze the PCR products. The gels and buffer systems are available from Pharmacia, Heidelberg or ETC, Kirchentellinsfurt. Different DNA electrophoresis gels are provided. For these applications 15% gels with 36 slots and a probe volume of 10–15 µL have proven to be useful, but other gel systems can also be used. Here we describe a native electrophoresis in a discontinuous buffer system (DELECT) with 15% gels which are recommended for the separation of small oligonucleotides. The DELECT buffer Kit contains a rehydration buffer for dry gels and a cathode and an anode buffer

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Table 2 Mixture for the Random Amplification of cDNA 10× Taq buffer 25 mM MgCl2 10 mM dNTPs 5 U/mL of Taq Long Plus ddH2O Primer (18 nM) cDNA (ca. 0.25 ng/µL)

5 µL 2 µL 10 µL 0.2 µL 20.8 µL 2 µL 10 µL

Table 3 PCR Conditions for the Random Amplification Phase

Temperature (°C)

Time

Cycles

94 94 36 72 94 50 72 4

5 min 30 s 30 s 1 min 30 s 30 s 1 min ∞

—

Denature Anneal Extend Denature Anneal Extend Cool

1

35 —

for electrophoresis. The following descriptions are always for whole gels of 36 slots. The gels can also be divided according to the number of probes, but the volumes and running conditions then have to be adapted.

3.4.1 Sample Dilution Four to six microliters of the sample should be diluted 1:1 in loading buffer for acrylamide gel electrophoresis.

3.4.2. Rehydration of the Dry Gel For rehydration of the gels special chambers are available, but any plain chamber could be used. 1. Pipet 35 mL of rehydration solution in the chamber and lay the gel film — with the gel surface facing down — into the buffer, avoiding air bubbles. 2. Shake the gel slowly for 90 min at room temperature. 3. Remove excess buffer with an absorbant paper (e.g., Whatman paper), dry the sample wells, and wipe buffer off the gel surface with the edge of the filter paper until you can hear a “squeaking.”

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200 V 375 V 450 V

20 mA 30 mA 30 mA

10 W 20 W 20 W

3.4.3 Application of the Gel and the Electrode Wicks 1. Switch on the thermostatic circulator of the Multiphor II chamber or of a comparable electrophoresis system; adjust to 21°C. 2. Lay two electrode wicks into the compartments of the paper pool, or any comparable chamber. 3. Apply 20 mL of the electrode buffer to each wick (for the anode wick use the anode buffer and for the cathode wick the cathode buffer). 4. Apply a very thin layer of kerosine onto the cooling plate with a tissue paper, to ensure good contact with the gel. 5. Place the gel (surface up) on the center of the cooling plate: the side containing the wells must be oriented toward the cathode. 6. Place the cathodal strip onto the cathodal edge of the gel, and the anodal one onto the other edge. For the DELECT buffer system the strips should be ca. 8 mm away from the edges of the samples. Smooth out any air bubbles.

3.4.4. Sample Application and Gel Electrophoresis 1. Apply 10–15 µL of each sample to the sample wells. To determine the size of the amplifed products it would be worthwhile in addition to apply a molecular weight marker. 2. Clean palladium electrode wires with a wet tissue paper before each electrophoresis run. 3. Move electrodes so that they will rest on the outer edges of the electrode wicks. The running conditions for a whole 36-slot 15% gel are shown in Table 4. If gels are divided the voltage should be kept constant and the mA and W should be adjusted according to the gel size (e.g., the half for half-gels). Depending on the fragment size, the second step has to be varied in length. The larger the amplified oligonucleotides are, the longer this step has to run. The xylene cyanol runs at a molecular range of ca. 100 basepairs. 4. After running, gels have to be fixed in 10% acetic acid or 15% EtOH/5% acetic acid for at least 30 min. The fixation step can be prolonged overnight.

3.4.5. Silver Staining 1. After fixation, gels are washed 3 × 5 min in ddH2O. Then the silver staining protocol shown in Table 5 is performed (see also Note 4).

Gels can be stored at room temperature after they are shrink-wrapped into a strong cling film.

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Table 5 Silver Staining Protocol of a Whole 15% 36-Slot CleanGel 20–30 min

200 mL

Stop/desilver

0.1% AgNO3 + 200 µL Formaldehyde (37%) Thoroughly wash gel surface with ca. 250 mL of ddH2O with a squeeze bottle 2–5 min 200 mL 2.5% Na2CO3 + 200 µL of (visual control) Formaldehyde (37%) + 200 µL of Na thiosulfate (2%) 10 min 250 mL 2.0% (g/v) glycine

Impregnate

10 min

250 mL

Silver reaction Washing Developing

10% Glycerol

Table 6 PCR Conditions for the Reamplification of Differentially Expressed Oligonucleotides Phase Denature Denature Anneal Extent Cool

Temperature (°C)

Time (min)

No. of cycles

94 94 55 72 4

5 1 2 1 ∞

— 35 —

3.5. Reamplification of the differentially expressed oligonucleotides 1. The differentially expressed bands can be cut out of the acrylamide gel with a scalpel and then reduced to small pieces with a pipet tip. 2. From 50 to 100 µL of sterile ddH2O is added and the probes then incubated for 30 min at 65°C and overnight at room temperature to elute the DNA out of the gel. 3. For the reamplification reaction, 10 µL of a 1:10 dilution of the eluted oligonucleotides are used. The PCR mixture is the same as in the amplification reaction. The conditions are shown in Table 6. 4. The PCR probes can be analyzed in a regular horizontal DNA electrophoresis with 1.5–2% agarose gels and then detected with an DNA intercalating stain (e.g., ethidium bromide). They also can be stored at –20°C.

The reamplification reaction could amplify multimers of the original oligonucleotide or other bands, which are artefacts from the elution of the acrylamide gel. For further cloning the fragments into a plasmid, only the interesting band should be eluted from an agarose gel by a convenient method. The cloned fragments should further be sequenced, because one eluted band from the random

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amplification might contain some fragments of the same size, but with different sequences. This is based on the randomly amplified products in the PCR, which can result in oligonucleotides coincidentally of the same size. The differential expression of the DNA fragments has to be confirmed, i.e., by Northern blotting (see also Note 5).

3.6. DNA Gel Electrophoresis 1. Agarose gels are prepared with 1× TBE buffer and should have a percentage of 1.5–2.0% agarose. 2. DNA gel electrophoresis is done in a horizontal manner. The gels have to be covered with running buffer (1× TBE). DNA probes are diluted 1:5 (v/v) in sample buffer and loaded into the sample wells of the gels (depending on the concentration and the volume of the gel wells, 10–40 µL of the DNA probe can be taken). To determine the size of the products it would again be useful to apply additionally a molecular weight marker. The orientation of the electrodes has to be chosen so that the DNA can run in the direction of the anode (positively charged electrode), because DNA is negatively charged. The voltage should be set to 1–15 V/cm (distance between the electrodes). The tracking dyes incorporated into the sample buffer serve as markers for the progress of the run. 3. At the end, the DNA is stained with ethidium bromide (0.5 µg/mL) for 2–5 min in a staining bath (Caution: wear gloves, because ethidium bromide is carcinogenic). After staining, the gels are briefly washed in ddH2O to remove the surplus ethidium bromide (again wear gloves and collect ethidium bromide waste separately). The DNA bands can be detected in UV light and can be documented with a suitable system (see also Note 7).

3.7 Purification of Nucleic Acids 3.7.1. Phenol/Chloroform Extraction Phenol is also a carcinogen, so work under a fume hood. The extraction of aqueous nucleic acids with phenol-chloroform allows the denaturing and removal of proteins (eg., enzymes) from the probes. 1. PCR probes (45–40 µL, see also Notes) are diluted with ddH2O to a final volume of 300–500 µL in a microfuge tube. 2. The same volume phenol/chloroform/isoamyl alcohol (25:24:1) is applied and probes are mixed vigorously on a vortex mixer. 3. To separate the aqueous—DNA containing—and the nonaqueous phase the probes are centrifuged at 12,000g for 2 min at room temperature. 4. The upper aqueous phase is transferred into a new microfuge tube and the same volume chloroform/isoamyl alcohol (24:1) is added. 5. The probe is mixed and centrifuged again. The upper phase is carried over in a new microfuge tube and the DNA is concentrated with the protocol for ethanol precipitation.

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3.7.2 Ethanol precipitation This protocol can be employed should the sample require concentrating and desalting. 1. To precipitate the DNA, the probe is treated with 1:10 vol 3 M NaAc, pH 4.8, and 2–3 vol of cold absolute ethanol. Then it is incubated at least 30 min at –20°C. 2. The precipitated nucleic acids are then centrifuged (12,000g, 15 min, 4°C), the supernatant discarded, and the pellet dried at 40–50°C with open cover to remove the surplus alcohol. 3. The DNA pellet can then be diluted in an adequate volume of ddH2O (to be suitable for the cloning protocol take only 11.7 µL of ddH2O).

3.5.3 DNA Gel extraction To elute DNA fragments from the agarose gels many kits are available from several companies. One of the most convenient is the QIAquick Gel Extraction kit (Qiagen, Hilden). The band of interest can be cut out of the gel under a UV transilluminator with a clean, sharp blade (wear glasses to protect your eyes). The fragment is now transferred into a clean microfuge tube and the extraction can be performed according to the protocol given.

3.8. Quantification by photometry Absorption at 260 nm is measured in an appropriate photometer to determine the concentration of nucleic acids. An absorption of 1 corresponds to 50 µg/mL double-stranded DNA, 40 µg/mL single-stranded RNA, and 30 µg/mL single-stranded oligonucleotides. The purity of the probes is determined with the quotient of the OD (optical density) at 260 nm:280 nm. This quotient should not be less than 1.8. 3.9. Cloning of PCR Fragments 3.9.1. Klenow Polymerase Treatment The 5':3' polymerase activity and the 3':5' exonuclease activity of the Klenow fragment of the polymerase I from E. coli is used to blunt the ends of the PCR fragments. 1. The DNA pellet from the ethanol purification is diluted in 11.7 µL of ddH2O. The following reagents are then added: 1.5 µL of 10× Klenow-buffer 1.8 µL of Klenow enzyme (0.8 U/µg of DNA) and the probe is incubated for 10 min at 37°C. 2. After that 1.2 µL of dNTPs (1.25 mM each nucleotide) are applied and a further incubation step at 37°C for 30–35 min is performed. 3. Heating at 70°C for 10 min denatures the Klenow fragment.

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3.9.2. 5'-Phosphorylation of the Fragments After the blunting of the fragments the 5'-ends have to be phosphorylated before they can be cloned into a plasmid vector. The enzyme T4-polynucleotide kinase (PNK) is employed for this purpose. 1. Add to the probe of 3.9.1: 2 µL of ATP (10 mM) 1 µL 10 × PNK buffer 1 µL of PNK (final volume: 20 µL) An incubation step of 30 min at 37°C is performed. 2. At this stage the PNK can be inactivated for 10 min at 70°C and the probes can be stored at –20°C. 3. Before cloning the probes have to be purified by agarose gel electrophoresis. The fragments of interest can then be separated via the gel from additionally amplified fragments and primer dimers from the reamplification reaction.

3.9.3. Preparation of the Plasmid Vector 1. The plasmid vector (e.g., pUC18, pUC19, Bluescript, or any other vector) is digested with a blunt ending restriction enzyme that cuts the plasmid vector into the multiple cloning site (MCS; i.e., SmaI) (see also Note 6). The restriction conditions depend on the enzyme used and should be followed according the product information. The volume should be between 20 and 50 µL and 2 U of the enzyme per microgram DNA should be added. After the incubation the enzyme has to be inactivated. It is mostly possible to heat inactivate the restriction enzymes, but if this should not be the case (see the product information) an agarose gel purification or ethanol precipitation has to be done. The purification of the vector from the restriction enzyme and the buffers is also necessary prior to the following dephosphorylation reaction. 2. To avoid the religation of the vector in the ligation reaction, it has to be dephosphorylated with an alkaline phosphatase. The dephosphorylation reaction is done with 0.04 U alkaline phosphatase/µg DNA in 1× phosphatase buffer (provided by the manufacturer) at 37°C for 60 min. After the incubation the vector has to be purified in an agarose gel and eluted with a suitable method.

3.9.4. Ligation For the ligation reaction there are many kits provided by several companies. We have had good experience with the Rapid Ligation Kit (Boehringer, Mannheim). This is a method for cloning blunt ended fragments into plasmid vectors. But of course any other ligation method, without using a kit, could be successful (see also Notes 8 and 9). A ligation procedure without using a kit requires the following reaction mix:

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Dephosphorylated, restricted plasmid (50 ng)/phosphorylated, blunt ended fragment in a ratio 1:3 to 1:5 1× Ligase buffer 1 µL of T4-ligase (5 U/µL) (total volume 20–40 µL)

The probe can then be incubated at 4°C overnight or at 16–20°C for 3–4 h. The probes can be stored at –20°C.

3.9.5. Preparation of CaCl2 competent cells for transformation The E. coli cells are treated following the CaCl2 procedure from Mandel and Higa (13). 1. 50 mL of LB medium are inoculated with 5 mL of an overnight preculture of the E. coli strain. The culture is then incubated at 37°C to an OD (600 nm) from 0.3 to 0.4. 2. The cells are centrifuged (2500g, 10 min, 4°C) and the pellet is resuspended in 100 mL of ice-cold 50 mM CaCl2 solution. 3. The cells are centrifuged again (only 1800g, 4°C). 4. The pellet is resuspended carefully in 20 mL of CaCl2 and incubated for 20 min on ice. 5. The cells are centrifuged again (1800g, 4°C) and the pellet is then carefully resuspended in 10 mL of ice-cold CaCl2 solution. In addition, glycerine is added to a final concentration of 20%. The probes should be divided in aliquots of 200 µL in microfuge tubes and then they can be stored at –80°C for several months.

3.9.6. Transformation 1. Prior to transformation, the ligation reaction mixture is filled with ddH2O to a final volume of 100 µL. 2. This mixture is added to a 200-µL aliquot of not completely thawed competent bacteria. The probe is then incubated 30 min on ice. 3. After that the bacteria are heat shocked for exactly 45 s at 42°C and 1 mL of 37°C warm LB medium is added at once. 4. The bacteria are incubated for 30–40 min. (They should have the possibility to divide once the plasmid incorporation is stabilized.) 5. Then aliquots of, e.g., 50, 100, and 200 µL are plated onto an LB agar plate with appropriate antibiotics or other selection markers (e.g., β-galactosidase). 6. The plates are incubated at 37°C overnight and can be stored at 4°C for a few days. Positive clones can be inoculated into LB medium with a suitable antibiotic and after an overnight incubation at 37°C (shake) the plasmids can be isolated for further control.

3.9.7. Plasmid Isolation by Alkaline Lysis For plasmid isolation, many reaction kits are available that can result in extremely pure plasmids. These are required, for example, for sequencing but

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for the control of plasmid transformation a simple protocol without using a kit, as described here, is enough. 1. From 1 to 3 mL of an overnight bacteria culture, transformed with a plasmid vector, are pelleted in a microfuge tube by centrifugation (full speed, 3 min). 2. The supernatant is discarded and the pellet is resuspended in 200 µL of cell resuspension solution by pipetting. 3. Next, 200 µL of cell lysis solution are added and mixed by inverting. The solution should clear almost immediately. 4. Then 200 µL of neutralization solution are added and the probes are again mixed by inverting (not by pipetting). 5. Spin down at 12,000g for 10 min. 6. Transfer supernatant to new tube. Discard pellet. 7. Add 1/10 of 1 volume of 3 M sodium acetate, pH 7.0 and 2.5 volumes of ETOH. 8. The probes are centrifuged at full speed for 5 min and the supernatant is discarded. 9. The pellet should be washed in 300 µL of 70% ethanol and finally dried with open cover at 40–50°C. 10. Then the pellet can be resuspended in 20–30 µL of ddH2O and stored at –20°C.

The ligation can be checked by a restriction digestion of the plasmid with two enzymes that each cut at one end of the insertion site of the fragment in the multiple cloning site of the plasmid vector. In a common agarose gel electrophoresis the genuine positive plasmids can be detected.

3.10. Verification of the Differentially Amplified Fragments in Northern Blot Analysis 3.10.1. RNA Gel Electrophoresis with a Denaturating Formaldehyde Gel 1. To prepare a 1–1.5% formaldehyde gel add 1–1.5 g of agarose into 73 mL of ddH2O. The agarose is dissolved by boiling and then cooled in a water bath to 50°C. The volume should be restored with ddH2O to 73 mL. Then 10 mL of 10× MOPS buffer and 17 mL of formaldehyde (37%) are added and the solution is mixed immediately and placed into a gel track. The fully polymerized gel can be applied into a horizontal gel electrophoresis tray and covered with RNA running buffer (1× MOPS buffer). 2. From 2 to 10 µg RNA is denatured 5 min at 65°C in the following mixture: RNA in a final volume of 6 µL (diluted in RNase-free ddH2O) 12.5 µL of formamide (deionized) 2.5 µL of 10× MOPS 4 µL of formaldehyde (37%) 3. The probe is then chilled on ice and 1:10 sample buffer is added. The probes are applied to the gel. The running conditions can be varied from 100 to 120 V for 3 h or overnight at 20 V. 4. The RNA run can be documented after ethidium bromide staining as for DNA.

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Fig. 1. Northern blotting setup.

3.10.2. Northern Blotting Here we describe a common method for Northern blotting that is performed with the ordinary capillary flow system. As RNA is electrophoresed in the denaturing system the gel does not need any further denaturation or neutralization. The RNA is transferred from the gel onto a positively loaded nylon membrane in a chamber construction shown in Fig. 1. The blotting setup is constructed in the following manner: 1. Set up the transfer support into the tray and place a wider glass plate onto the support. Put two pieces of Whatman 3MM paper on the glass plate so that they hang up into the tray. Fill the reservoir with transfer buffer (20× SSC) and wet also the Whatman 3MM paper (avoid air bubbles). 2. Cut a piece of the nylon membrane of the same size as the gel (handle the filter at the edges only!) and wet the filter for 5 min in the transfer buffer. Cut off the lower right corner of the filter (so as not to lose orientation during the procedure). 3. Flood the two Whatman 3MM papers on the glass plate with transfer buffer and place the gel onto the papers. Squeeze out air bubbles by rolling with an RNasefree pipet over the gel. Cut a piece of the size of the gel out of the middle of a piece of common cling film and apply this around the gel. This will ensure that the liquid flow from the reservoir is transferred through the gel. 4. Apply the nylon membrane carefully directly onto the gel and again squeeze out air bubbles as described previously.

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5. Wet two or three pieces of Whatman 3MM paper of the same size as the gel and place them onto the membrane. Cut paper towels to the same size and put them onto the Whatman papers to form a stack 7–8 cm high. 6. Apply a glass plate onto the top and a weight of 400–500 g onto it to ensure good contact during the transfer. 7. The RNA is transferred overnight from the gel to the nylon membrane at room temperature. 8. After blotting, the RNA is fixed by incubating the membrane for 5 min onto a Whatman paper that is soaked with 0.05 M NaOH (RNA site up). Before storing the filter at 4°C in cling film it should be briefly washed with 2× SSC solution.

3.10.3. Probe Hybridization and Labeling Here we describe a method which uses the standard megaprime protocol from the Megaprime™ labeling kit (Amersham) (see also Notes 10 and 11). 1. From 2.5 to 25 ng of the DNA are dissolved in 10 µL of ddH2O. Twenty-five nanograms of the template DNA are then applied into a microfuge tube and 5 µL of the random primer are added. 2. The DNA is denatured at 95–100°C for 5 min in a boiling water bath. 3. The probe is collected by a short centrifugation step and then kept at room temperature. 4. To label the DNA template the following components are added: Unlabeled dNTPs 4 µL each of dCTP, dTTP, dGTP; 2.5 mM 5 µL of reaction buffer Radiolabeled dATP 5 µL of [32P]dATP (specific activity; 3000 Ci/mmol] 2 µL of DNA polymerase I (at –20°C until needed) 11 µL of ddH2O 5. The probe is mixed gently and incubated at 37°C for 10–30 min. 6. The reaction is stopped by heating the probe at 95°C for 5 min. The surplus [32P]ATP can be removed on a Sephadex G50 column. 7. The probe can now be stored at –20°C for 2–3 d. Further storage should be avoided because of the short half-life of 32P. 8. Prior to hybridization, the membrane is incubated for at least 15 min at 65°C or overnight in a prehybridization box with prehybridization solution. 9. For the hybridization, the labeled DNA probe is denatured by heating to 95–100°C for 5 min, and then directly chilled on ice. The labeled DNA probe can now be added into the prehybridization solution at a concentration of 1 × 106 cpm/mL and incubated at 55–65°C for at least 12 h (the higher the hybridization temperature chosen, the higher the stringency of the hybridization). 10. After hybridization, the filters are washed by incubating for 5 min at 65°C. This procedure is repeated if necessary and checked with a hand monitor for radioactive detection (Geiger counter). The membrane is now wrapped in a nylon filter (e.g., Saran Wrap) and the hybridization is detected by autoradiography.

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Table 7 Primer Sequence of a β-actin gene that Gives Bands of Different Sizes for cDNA and gDNA Target

Primer sequence (5' → 3')

Product (bp)

β-Actin sense β-Actin antisense

GGC GGC ACC ACC ATG TAC CCT AGG GGC CGG ACT CGT CAT ACT

202 (312)

4. Notes 1. Working with RNA demands some special precautions, as RNA is rapidly degraded in the presence of the very stable enzyme RNase. The possibility of contamination with RNase should be minimized. Therefore always wear gloves when preparing or handling RNA probes. The laboratory equipment for RNA procedures should be separated if possible or sterilized in an autoclave before use. Equipment that cannot be autoclaved should be treated with 0.5 M NaOH or 70% ethanol. RNase can be inactivated with DEPC (CAUTION: carcinogen; use a fume hood). DEPC should be added to every solution in a final concentration of 0.1% (but not to solutions that contain Tris) and then incubated overnight in a fume hood with cover open, or if possible autoclaved for 20 min. For RNA preparation, a method for isolating cytoplasmic RNA is useful, to avoid amplification of incompletely spliced RNA in the reverse transcription. Genomic DNA contamination is one of the major causes of false-positive results. Although most kits guarantee preparation of DNA-free RNA, it would be useful to check samples for possible DNA contaminations. This could be done by an RT-PCR reaction with a primer to genes usually expressed that gives different products with mRNA and gDNA (e.g., Table 7). In the RNA gel electrophoresis, the formamide that inhibits the RNases can be reduced by half if the additional volume is needed. 2. It is useful to confirm the success of the reverse transcriptase reaction in a simple common PCR reaction with primers to genes usually expressed in the material of interest. 3. The use of too much cDNA in the amplification reaction, or in the reamplification, can have a bad effect on the reaction. In the worst case, there may be no amplification of the DNA at all. The various bands of the acrylamide gel should be cut out as small as possible, to avoid contamination with neighboring bands. 4. Silver staining is a very sensitive method for oligonucleotide detection. To be able to differentiate varying bands, the probe dilution in the acrylamide gel electrophoresis has to be optimal. Our experience has shown that 5–8 µL from the amplification reaction are enough to detect clear bands. The developing reaction in silver staining can happen very quickly. To avoid the gel becoming too dark it is useful to prepare the stop/desilver solution before-

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5.

6.

7. 8. 9.

10.

11.

Engel, Adibzadeh, and Pawelec hand, so that the gel can be transferred rapidly into the stop bath once the reaction has progressed far enough. Before cloning, 5–10 µL of the reamplified fragment should be checked in an agarose gel and the DNA content should be measured. The other 40–45 µL can then be treated with the chloroform phenol extraction protocol. The enzyme content of the restriction probe should not be more than 1:10 of the entire volume, because the enzymes are diluted in a very highly concentrated glycerol solution that inhibits the enzyme activity. Before running an agarose gel, incubate the isolated plasmid vector with 1 U of RNase/20 µL of probe for 10 min at 37°C to remove RNA contamination. Avoid too much pipetting after the ligase is added, because this enzyme is sheared very easily and thereby loses its activity. A satisfactory and very uncomplicated ligation kit is provided by Boehringer Mannheim. The ligation protocol can be followed as described but the incubation time can be extended to 30–45 min instead of the 5 min given in the manufacturer’s directions for use. The protocol that is described uses a laboratory equipped for radioactive methods! It would be useful to prepare the radioactively labeled probes in microfuge tubes that have a cover with a screw thread to minimize risk of aerosols upon opening the tubes after heating. The polymerase should be added last, because the reaction immediately starts when the enzyme is applied. If one dNTP is missed, every amplification will stop at the complementary nucleotide of the missing nucleotide. The membrane should not be washed for too extended a period, because the labeled probes would be washed away. After that the membrane can be hybridized again with another probe. This process can be repeated up to 4–5 times.

References 1. Linskens, M. H., Feng, J., Andrews, W. H., Enlow, B. E., Saati, S. M., Tonkin, L. A., Funk, W. D., and Villeponteau, B. (1995) Cataloging altered gene expression in young and senescent cells using enhanced differential display. Nucleic Acids Res. 25, 3244–3251. 2. Swisshelm, K., Ryan, K., Tsuchiya, K., and Sager, R. (1995) Enhanced expression of an insulin growth factor-like binding protein (mac 25) in senescent human mammary epithelial cells and induced expression with retinoic acid. Proc. Natl. Acad. Sci. USA 92, 4472–4476. 3. Salehi, M., Hodkins, M. A., Merry, B. J., and Goyns, M. H. (1996) Age-related changes in gene expression in the brain revealed by differential display. Experientia 15, 888–891. 4. Wu, H. C. and Lee, E. H. (1997) Identification of a rat brain gene associated with aging by PCR differential display method. J. Mol. Neurosci. 8, 13–18. 5. Liang, P. and Pardee, A. B. (1992) Differential display of eucaryotic messenger RNA by means of the polymerase chain reaction. Science 257, 967–971.

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6. Bauer, D., Müller, H., Reich, J., Riedel, H., Ahrenkiel, V., Warthoe, P., and Strauss, M. (1993) Identification of differentially expressed mRNA species by an improved display technique (DDRT-PCR) Nucleic Acids Res. 21, 4272–4280. 7. Liang, P., Bauer, D., Averboukh, L., Warthoe, P., Rohrwild, M., Müller, H., Strauss, M., and Pardee, A. B. (1995) Analysis of altered gene expression by differential display. Methods Enzymol. 254, 304–321. 8. Welsh, J., Chada, K., Datal, S. S., Cheng, R., Ralph, D., and McClelland, M. (1992) Arbitrary primed PCR fingerprinting of RNA. Nucleic Acids Res. 20, 4965–4970. 9. Welsh, J., Rampino, N., McClelland, M., and Perucho, M. (1995) Nucleic acid fingerprinting by PCR-based methods: applications to problems in aging and mutagenesis. Mutat. Res. 338, 215–229. 10. McClelland, M. and Welsh, J. (1994) RNA fingerprinting by arbitrary primed PCR. PCR Methods Appl. 4, 66–81. 11. Liang, P., Averboukh, L., and Pardee, A. B. (1994) Method of differential display. Methods Mol. Genet. 5, 3–16. 12. Sambrook, F., Fritsch, E. F., and Maniatis, T. (1989) Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York. 13. Mandel, M. and Higa, A. (1970) Calcium dependent bacteriophage DNA infection. J. Mol. Biol. 53, 159–162. 14. Wei, Q., Xu, X., Cheng, L., Legerski, R. J., and Ali-Osman, F. (1995) Simultaneous amplification of four DNA repair genes and β-actin in human lymphocytes by multiplex reverse transcriptase PCR. Cancer Res. 55, 5025–5029.

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8 Xenobiotic-Metabolizing Enzyme Systems and Aging Christopher R. Barnett and Costas Ioannides 1. Introduction The human body is continuously exposed to a wide array of structurally diverse chemicals. Such exposure occurs even at the fetal stage as almost all chemicals that are present in the mother’s blood can readily cross the placenta and reach the fetus. Some of these chemicals are ingested voluntarily, for example, medicines and food additives, but the vast majority are taken involuntarily, as environmental contaminants present in the air or in the occupational environment. Undoubtedly, the most important source of such chemicals is the diet, and many dietary constituents have been shown to induce many forms of toxicity including cancer (1). Exposure to chemicals is thus inevitable and unavoidable. The body cannot exploit these chemicals either to generate energy or transform them to building blocks and consequently its response is to rid itself of their presence. This chapter discusses the role of drug-metabolizing enzyme systems in this process and the effects of age. The measurement of drug-metabolizing activities is of increasing importance in the safety evaluation of drugs in humans. This chapter describes the use of alkylphenoxazone derivatives for investigating selected activities of drug-metabolizing enzymes. Chemicals that reach the systemic circulation and are distributed throughout the body are generally lipophilic, a property that allows them to traverse the various cellular membranes. Such lipophilic chemicals are also difficult to excrete through the kidney and bile. Consequently, to facilitate their elimination, the body converts them to hydrophilic metabolites, which are much easier to excrete. Furthermore, such metabolism terminates any biological activity that may be manifested by these chemicals, as the products of metabolism are biologically inactive, being unable to interact with the receptors for which the From: Methods in Molecular Medicine, Vol. 38: Aging Methods and Protocols Edited by: Y. A. Barnett and C. R. Barnett © Humana Press Inc., Totowa, NJ

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Fig. 1. Metabolic activation of chemicals.

parent compounds often have high affinity. The metabolism of chemicals occurs through an enzymic process that involves a number of enzyme systems present in many tissues, the highest concentration being encountered in the liver. The metabolism of chemicals is generally achieved in two phases. Phase I involves primarily the incorporation of an atom of oxygen to the substrate, producing a more hydrophilic, and in most cases biologically inactive, metabolite that can now participate in Phase II metabolism. During Phase II metabolism, the newly generated metabolite is conjugated with endogenous substrates, such as sulfate and glucuronide, to form highly hydrophilic products that can now be very readily eliminated.

1.1. Metabolic Activation of Chemicals Although metabolism of chemicals is essentially a deactivation process, with certain chemicals one or more metabolic pathways may lead to the generation of reactive intermediates, a phenomenon known as metabolic activation or bioactivation (Fig. 1). Because of their increased chemical reactivity,

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these intermediates interact covalently with vital cellular components, leading to the manifestation of toxicity (2,3). For example, most chemical carcinogens are metabolized to reactive species that interact with DNA to induce mutations that may lead to the formation of tumors. Generally, the activation pathways are usually minor routes of metabolism and the body is well equipped to deal with the limited amounts of reactive intermediates that are produced. This defense mechanism proceeds through conjugation of the reactive intermediates with the endogenous tripeptide glutathione, the glutathione conjugates being excreted in the urine and feces following additional processing that occurs principally in the kidney and intestine. It is therefore not surprising that the cellular concentration of glutathione in the hepatocyte, the major site of the bioactivation of chemicals, is high (about 10 mM). Depletion of the tissue stores of glutathione, whether by chemicals or as a consequence of poor nutrition, can potentiate markedly chemical toxicity. The toxicity of the mild analgesic and antipyretic drug paracetamol (acetaminophen) is markedly exacerbated if the animals have been depleted of glutathione as a result of inadequate nutrition (4). It is evident that a chemical is subject to metabolism through a number of pathways, most of which will result in deactivation. Certain routes of metabolism, however, will produce deleterious intermediates that are themselves subject to deactivation through metabolism. Clearly, the amount of reactive intermediates available for interaction with cellular components, and hence the degree of toxicity, is largely dependent on the competing pathways of activation and deactivation. If an animal species favors the activation pathways of a chemical it will be susceptible to its toxicity whereas if deactivation is favored it will be resistant. 2-Aceylaminofluorene is a carcinogen that undergoes N-hydroxylation to generate the reactive, carcinogenic intermediates. The guinea pig is unable to catalyze this reaction and consequently it is very resistant to the carcinogenicity of this chemical (5). Clearly, toxicity is not simply a consequence of the intrinsic molecular structure and physicochemical properties of the chemical, but is also largely dependent on the nature and level of the enzymes present in the living organism at the time of exposure. Normally, the activation pathway is a minor route of metabolism, but under certain circumstances it may assume greater importance. Such a situation arises when the deactivation pathways are saturated, as a result of intake of high doses of the chemical or when the activation pathway is selectively induced, for example, as a result of prior exposure to chemicals. Under such circumstances, the production of reactive intermediates is stimulated, overwhelming the deactivation pathways, making an interaction with cellular constituents, and the ensuing toxicity, more likely. Paracetamol is a very safe drug, the activation pathway being a very minor metabolic route. In alcoholics, as a result of alcohol intake,

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this pathway is more active so that they may suffer hepatotoxicity even when consuming therapeutic doses of the drug (6).

1.2. The Cytochrome P450-Dependent Mixed-Function Oxidase System Many enzyme systems participate in the metabolism of chemicals, both Phase I and Phase II. In Phase I metabolism, undoubtedly the most important are the cytochrome P450-dependent mixed-function oxidases, a ubiquitous enzyme system found in almost every tissue, the highest concentration being encountered in the liver (7). It is a very versatile enzyme system, capable of metabolizing structurally very diverse chemicals of markedly different molecular shape and size. There are not many chemicals that find their way in the human body and are not metabolized, at least to a small extent, by the cytochrome P450 system and many are metabolized exclusively by this system. It has evolved to deal with the increasing number of xenobiotics to which humans are exposed. In addition to metabolizing xenobiotics, this enzyme system also makes a major a contribution to the metabolism of endogenous substrates such as steroid hormones, vitamins, fatty acids, and prostaglandins. The cytochrome P450 system comprises an electron transport chain consisting of the flavoprotein cytochrome P450 reductase and the hemoprotein cytochrome P450, which acts as a terminal oxidase. It catalyzes the incorporation of one atom of molecular oxygen to the substrate while the second atom forms water. Cytochromes P450 achieve their broad substrate specificity by existing as a superfamily of proteins, divided into families on the basis of their structural similarity. Each family may be subdivided into subfamilies that may contain one or more proteins. For example, the cytochrome P450 family one (CYP1) comprises two subfamilies, namely A (CYP1A) and B (CYP1B). The CYP1A subfamily consists of two proteins (isoforms), CYP1A1 and CYP1A2. Only a single protein belongs to the other subfamily (CYP1B1). Of the cytochrome P450 families, the most important contributors to xenobiotic metabolism are CYP1–CYP3, and the major characteristics of these are summarized in Table 1. The CYP1 family appears to be the most important in the bioactivation of chemicals (8,9).

1.3. Regulation of Cytochromes P450 It has long been appreciated that humans differ markedly in the way they respond to the same chemical exposure. Dramatic interindividual differences have been noted in the blood levels of drugs following the intake of the same dose, which could account for the fact that they experienced different pharmacological effects, a number developing adverse effects commensurate with overdosage. It has now been recognized that individuals may metabolize the same drug at different rates, reflecting the activity of their drug-metabolizing enzymes,

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Table 1 Principal Characteristics of the Xenobiotic-Metabolizing Cytochrome P450 Enzymes Family

Subfamily

CYP1

A B A B C D E A

CYP2

CYP3

Typical drug substrate Theophylline Theophylline Propranolol Warfarin Tolbutamide Debrisoquine Chlorzoxazone Erythromycin

Role in bioactivation

Inducibility by chemicals

Very extensive Very extensive Limited Limited Poor Poor Extensive Limited

Very high High Modest High Modest Not inducible High High

particularly the cytochromes P450. Many factors are responsible for the marked interindividual drug-metabolizing activity, including genetic background, nutritional status, presence of disease, and previous exposure to other xenobiotics. Undoubtedly one of the factors that govern cytochrome P450 activity is genetic makeup. Indeed, it was established some two decades ago that cytochrome P450 isoforms may be polymorphically expressed. This realization followed observations that some persons, about 5–10% of Europeans, displayed exaggerated responses after the intake of therapeutic doses of the antihypertensive drug debrisoquine. This drug is normally deactivated through 4-hydroxylation but the poor metabolizers are unable to carry out this pathway because they lack a functioning CYP2D6, the cytochrome P450 enzyme catalyzing this reaction (10). The expression of cytochrome P450 activity is also regulated by the levels of endogenous hormones as well by disease and especially previous exposure to other chemicals, capable of inhibiting or inducing one or more cytochrome P450 proteins. A number of studies established the importance of hormones such as androgens, growth hormone, and thyroid hormone in the regulation of cytochrome P450 enzyme (11). Changes in the levels and patterns of excretion of hormones may result in selective modulation of cytochrome P450 proteins. For example, hyposecretion of growth hormone has been implicated in the alterations in the hepatic profile of cytochromes P450 observed in animals with insulin-dependent diabetes mellitus (12). Exposure to environmental chemicals, as well as many drugs, can up-regulate cytochrome P450 proteins in the liver and other tissues, so that chemicals ingested after induction has occurred will be more extensively metabolized if they rely on the induced enzymes for their metabolism. Human cytochrome P450 proteins have been shown to be induced by alcohol; consumption of cru-

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ciferous vegetables or of charcoal-broiled beef; smoking; and intake of drugs such as omeprazole, rifampicin, and phenobarbitone. Cytochrome P450 proteins can also be down-regulated following exposure to exogenous chemicals so that the metabolism of any subsequently ingested chemicals will be suppressed if they rely on the inhibited enzymes for their metabolism. This is very much highlighted in the recently recognized interaction between normal grapefruit juice and a number of drugs including dihydropyridine calcium channel blockers such as felodipine, nisoldipine, and nifedipine as well as of other drugs such as quinidine, midazolam, terfenadine, and cyclosporine (13). Simultaneous consumption of grapefruit juice with these drugs resulted in higher plasma levels than anticipated, leading to increased adverse effects. These drugs are extensively metabolized in the intestine by CYP3A4, which is effectively inhibited by grapefruit juice. Chemicals can have a differential effect on the expression of individual cytochromes P450. For example, the dietary chemical diallyl sulfide, a naturally occurring chemical in garlic, down-regulates CYP2E1 but up-regulates CYP2B in the liver of rats (14). Cytochrome P450 activity is also influenced by the presence of disease. Again the effects are selective, in that only certain isoforms are influenced, with some being depressed whereas others are stimulated. Hepatic disease such as hepatocellular carcinoma has been shown to perturb the profile of cytochromes P450 in patients with cirrhosis and hepatocellular carcinoma (15). In these situations the disease affects the liver itself, but hepatic cytochrome P450 levels may also be modulated in diseases where the liver is not the primary target of disease such as insulin-dependent diabetes mellitus (12).

1.4. Drug Metabolism in the Aged Although in animal studies drug-metabolizing activity has been reported to diminish in the old, the limited studies conducted in humans do not appear to support the view that age is an important determinant of drug metabolism activity. Although a number of drugs are poorly eliminated in the elderly, this does not necessarily reflect reduced metabolic, including cytochrome P450, activity. They may be secondary to the normal physiological changes that accompany old age such as decreased renal capacity to excrete drugs and their metabolites, reduced liver blood flow, decrease in liver mass, and changes in plasma protein levels, one or all of which may account for the impaired drug elimination. Both renal glomerular filtration and tubular function are altered in the aged without any signs of kidney dysfunction. Blood flow in the old may be as little as half of that of the adult, affecting particularly the elimination of drugs having a high extraction ratio, which is defined as the difference in the concentration of drug entering and coming out of the liver divided by the concentration of drug entering the liver. Plasma levels of albumin, the major pro-

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tein to which drugs bind, decrease in the aged, presumably the consequence of reduced synthesis, leading to lower protein binding. In animal studies, it was repeatedly shown that the metabolism of many xenobiotics declines in old age, resulting in prolongation of the pharmacological effect of drugs (16,17). However, evidence for reduced capacity in the metabolism of drugs through cytochrome P450 catalyzed pathways is lacking (18). It is also feasible that the various cytochrome P450 proteins respond differently to the onset of old age, but this remains to be investigated. In studies carried out in male rats, aged between 1 wk and 2 yr, cytochrome P450 activity was investigated by determining the hydroxylation of testosterone at different positions and was complemented by immunological determination of the apoprotein levels (19). It was evident that age-dependent changes differed among the cytochrome P450 enzymes studied. For example, hepatic levels of CYP2C11 disappeared in old age whereas CYP2A1 levels increased and those of CYP2E1 were unaffected. The recent availability of in vivo probes displaying selectivity for specific cytochrome P450 forms has made it possible to assess the effect of age on cytochrome P450 expression in humans. The N-demethylation of erythromycin, a diagnostic probe for CYP3A4, the major cytochrome P450 enzyme in the human liver, was determined by measuring the amount of carbon dioxide exhaled. No difference was detectable between healthy aged volunteers (age ranging between 70 and 88 yr) in comparison to younger adults (age ranging between 20 and 60), showing that the expression of CYP3A is not affected by age in humans (20). Similarly, the expression of CYP2E1 was constant in humans aged between 30 and 75 yr of age (21). In more recent studies, employing as probes lignocaine (CYP3A4) and coumarin (CYP2A6), a decrease was reported in the levels of these drugs with increasing age (22). In these studies the authors compared healthy young volunteers (65 yr). Similarly, in recent extensive studies, the half-life of antipyrine, a drug whose metabolism involves a number of cytochrome P450 proteins, increased in the elderly whereas its clearance decreased (23). Clearly, the effect of age on individual cytochrome P450 enzymes is far from being understood, and it is only now that such studies are being undertaken. It must be emphasized that the old consume a disproportionate number of drugs compared with other subpopulations and an understanding of their ability to handle drugs will lead to more effective treatment.

1.5. Measurement of Cytochrome P450 Activities in Human Liver Using Alkoxyresorufins In 1974, Burke and Mayer (24) demonstrated that an alkoxyphenoxazone derivative, ethoxyresorufin, could be metabolized by CYP1A1 with a high

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specificity. This specificity has been demonstrated for many animals species, including humans. Furthermore, the use of methoxyresorufin and pentoxyresorufin derivatives allow the measurement of CYP1A2 and CYP2B proteins with high selectivity. The metabolism of the alklyphenoxazone derivatives can be measured using microsomal or whole cell protein (26). Both methods are comparable except for the inclusion of dicoumarol in the whole cell protein method to prevent the cytosolic reduction of resorufin to a nonfluorescent molecule by NAD(P)H oxidoreductase.

1.6. Method for the Measurement of Ethoxy-, Methoxy-, and Pentoxyresorufin Dealkylation Using Human Liver Samples This method can be used for measurement of CYP1A1 (ethoxyresorufin), CYP1A2 (methoxyresorufin), and CYP2B (pentoxyresorufin) activities in microsomal protein fractions or cell homogenates from primary hepatocytes or cultured cells. It should be noted that ethyoxyresorufin may be deethylated to some extent also by CYP1A2. In human liver, expression of CYP1A1 is very low and ethoxyresorufin O-deethylase activity is largely attributable to CYP1A2, although a small contribution from other subfamilies such as CYP2C cannot be excluded. 2. Materials 2.1. Preparation of Hepatic Subcellular Fractions 1. Potter–Elvehjem glass–Teflon homogenizer (BDH, Poole, Dorset). 2. 1.15% (w/v) KCl, 4°C. 3. Refrigerated centrifuge capable of producing 9000g, ultracentrifuge.

2.2. Measurement of Cytochrome P450 Activities Using Alkoxyresorufins For the direct measurement of alkoxyresorufin O-dealkylase activity the method of Burke and Mayer (24) can be used. 1. Pentoxy-, ethoxy- and methoxyresorufin as well as resorufin can be obtained from Molecular Probes, Eugene, OR, USA. Alkoxyresorufins are dissolved in dimethylformamide (Sigma Chemical, Dorset, UK) to provide stock concentrations of 0.53 mM ethoxyresorufin, 1 mM pentoxyresorufin, and 0.53 mM methoxyresorufin. These solutions can be stored at –20°C until required and should be maintained in the dark at all times. Resorufin is also dissolved in dimethylformamide to produce a 0.1 mM stock solution. This fluorescent compound can be stored at –20°C in the dark until required. 2. Tris-HCl buffer (0.1 M, pH 7.8) prepared by dissolving 0.1 moles of Tris base (Sigma Chemical, Dorset, UK) in 850 mL of distilled water. Using a calibrated

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4. 5.

6.

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pH meter the pH of the buffer is adjusted to 7.8 using 3 M HCl. The buffer is transferred to a 1-L volumetric flask and made up to the 1-L mark with distilled water. The pH of the buffer solution is confirmed at pH 7.8 using a pH meter. NADPH (50 mM, Sigma Chemical, Dorset, UK) is dissolved in 1% (w/v) sodium hydrogen carbonate and kept at 4°C until required. This solution is made fresh prior to performing the assays. Dicoumarol (20 mM, Sigma Chemical, Dorset, UK) is prepared by dissolving dicoumarol in 0.1 M Tris-HCl buffer, pH 7.8. Spectrofluorometer with excitation wavelength of 510nm and emission wavelength of 586 nm with excitation and emission slit widths of 10nm and 2.5nm, respectively. Positive controls. Samples of rodent liver microsomes that have high activity for ethoxyresorufin, methoxyresorufin, or pentoxyresorufin can be obtained from Xentox Limited, Northern Ireland, UK.

3. Methods 3.1. Preparation of Hepatic Subcellular Fractions Microsomal fractions are prepared according to the method of Ioannides and Parke (26). 1. Liver sample is weighed and and transferred to a glass beaker with volume capacity at least 5× that of the weight of the liver sample, for example, 10 g of liver in a 50-mL beaker. 2. The sample is scissor-minced and transferred to the Potter–Elvehjem homogenizer together with 3× the liver weight of 1.15% KCl (4°C). 3. Homogenize the sample using several up-and-down strokes of the homogenizer. 4. The homogenate should be maintained at 4°C during the homogenization process using an ice jacket (metal can filled with ice surrounding the glass homogenizer). 5. The homogenate is transferred to a measuring cylinder and made up to 4× the initial sample weight with 1.15% (w/v) KCl, for example, 10 g of liver sample made up to 40 mL of final homogenate volume with 1.15% (w/v) KCl. This is a 25% w/v liver homogenate. 6. The homogenate is transferred to centrifuge tubes and the tubes balanced for centrifugation at 9000g for 20 min. 7. Following centrifugation at 9000g for 20 min in a refrigerated (4°C) centrifuge the supernatant (S9) is decanted and may be stored at –70°C for up to 6 mo. 8. For microsomal preparation the S9 is transferred to ultracentrifuge tubes and balanced for ultracentrifugation at 105,000g for 60 min at 4°C. 9. The supernatant (cytosolic fraction) is discarded and the pellet resuspendend in a volume of 1.15% w/v KCl equal to the volume of S9 initially placed into the ultracentrifugation tube. 10. The microsomal suspension should be kept on ice and used the same day.

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3.2. Measurement of Alkoxyresorufin Metabolism 1. The reaction is carried out at 37°C. To a 3-mL fluorimetric cuvet (a 1.5-mL microfluorimetric cuvet may be used with appropriate adjustment of volumes given below) add the following reagents: 1.935 mL of 0.1 M Tris-HCl buffer, pH 7.8, prewarmed to 37°C 50 µL of microsomal suspension/cell homogenate* 3 µL of 0.53 mM ethoxyresorufin or 5 µL of 0.53 methoxyresorufin or 5 µL of 1 mM pentoxyresorufin *If using cell homogenate (S9) then dicourmarol should be added to give a final concentration of 10 µM (substitute 10 µL of Tris buffer for 10 µL of dicoumarol stock solution). 2. The cuvet is introduced into the spectrofluorometer and a baseline recorded prior to initiation of the reaction with 10 µL of NADPH solution. 3. The reaction is monitored continuously until a measurable gradient is obtained and an initial rate of reaction can be determined. 4. Resorufin production from the alkoxyresorufin substrate can be calculated using the standard resorufin solution. 5. A blank is prepared by replacing the microsomes or cell homogenate with 50 µL of Tris-buffer. 6. Following the establishment of baseline fluorescence, at least three 10-µL samples of the standard resorufin are introduced into the cuvet, noting the increase in fluorescence after each addition.

3.3. Example Calculation 1. Ten microliters of 0.1 mM resorufin caused an increase of 15.5 fluorescence units. 2. Fifty microliters of sample A caused an increase of 1.2 fluorescence units per minute. 3. Therefore 1 mL of sample A would cause 1000/50 × 1.2 unit increase per minute or 24 units per minute. 4. Ten microliters of 0.1 mM resorufin is equal to 1nmole of resorufin, therefore 1 nmol of resorufin will cause a 15.5 unit increase in fluorescence. 5. As such 1 mL of sample A produced 24/15.5 nmol resorufin per minute = 1.5 nmol/min/mL. 6. Having established the protein concentration in the microsomal suspension/cell homogenate, the activity can be expressed as nmol/min/mg of protein.

Many spectrofluorometers will perform these calculations directly following the calibration step with the resorufin standard. 4. Notes 1. A baseline cannot be established as the fluorescence is increasing at a steady rate. The alkoxyresorufin substrate may have become contaminated with NADPH or S9. Make up fresh substrate and ensure that a separate pipet is used for each addition.

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2. No reaction appears to be occurring. Check reaction system using positive controls. Check fluorimeter settings. Ensure that buffer is warmed to 37°C. 3. No reaction seems to be occurring. Increase amount of microsomal or S9 suspension added and reassay.

References 1. Ioannides, C., ed. (1998) Nutrition and Chemical Toxicity. John Wiley & Sons, Chichester. 2. Hinson, J. A., Pumford, N. R., and Nelson, S. D. (1994) The role of metabolic activation in drug toxicity. Drug Metab. Rev. 26, 395–412. 3. Vermeulen, N. P. E. (1996) Role of metabolism in chemical toxicity, in Cytochromes P450: Metabolic and Toxicological Aspects (Ioannides, C., ed.), CRC Press, Boca Raton, FL, pp. 29–53. 4. Pessayre, D., Dolder, A., Arigou, J. Y., Wandscheer, J.-C., Descatoire, V., Degott, C., and Benhamou, J. P. (1979) Effect of fasting on metabolite-mediated hepatotoxicity in the rat. Gastroenterology 77, 264–271. 5. Kawajiri, K., Yonekawa, H., Hara, T., and Tagashira, Y. (1978) Biochemical basis for the resistance of guinea pigs to carcinogenesis by 2-acetylaminofluorene. Biochem. Biophys. Res. Commun. 85, 959–965. 6. Maddrey, W. C. (1987) Hepatic effects of acetaminophen. Enhanced toxicity in alcoholics. J. Clin. Gastroenterol. 9, 180–185. 7. Guengerich, F. P. (1993) Cytochrome P-450 enzymes. Am. Scientist 81, 440–447. 8. Ioannides, C. and Parke, D. V. (1990) The cytochrome P450 I gene family of microsomal haemoproteins and their role in their metabolic activation of chemicals. Drug Metab. Rev. 22, 1–85. 9. Gonzalez, F. J. and Gelboin H. V. (1994) Role of human cytochromes P450 in the metabolic activation of chemical carcinogens and toxins. Drug Metab. Rev. 26, 165–183. 10. Meyer, U. A., Skoda, R. C., and Zanger, U. M. (1990) The genetic polymorphism of debrisoquine/sparteine metabolism — molecular mechanisms. Pharmacol. Ther. 46, 297–308. 11. Westin, S., Tollet, P., Ström, A., Mode, A., and Gustafsson, J. Å. (1992) The role and mechanism of growth hormone in the regulation of sexually dimorphic P450 enzymes in rat liver. J. Steroid Biochem. Molec. Biol. 43, 1045–1053. 12. Ioannides, C., Barnett, C. R., Irizar, A., and Flatt, P. R. (1996) Expression of cytochrome P450 proteins in disease, in Cytochromes P450: Metabolic and Toxicological Aspects (Ioannides, C., ed.), CRC Press, Boca Raton, FL, pp. 301–327. 13. Ameer, B. and Weintraub, R. A., 1997, Drug interactions with grapefruit juice. Clin. Pharmacol. 33, 103–121. 14. Dragnev, K. H., Nims, R. W., and Lubet, R. A. (1995) The chemopreventive agent diallyl sulfide. A structurally atypical phenobarbital-type inducer. Biochem. Pharmacol. 50, 2099–2104. 15. Guengerich, F. P. and Turvy, C. G. (1991) Comparison of levels of several human cytochrome P-450 enzymes and epoxide hydrolase in normal and disease states

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using immunochemical analysis of surgical liver samples. J. Pharmacol. Exp. Ther. 256, 1189–1191. Schmucker, D. L. (1985) Aging and drug disposition in the elderly: an update. Pharmacol. Rev. 37, 133–148. Durnas, C., Loi, C.-M., and Cusack, B. J. (1990) Hepatic drug metabolism and aging. Clin. Pharmacokinet. 19, 359–389. Wynne, H. A., Mutch, E., James, O. F. W., Rawlins, M. D., and Woodhouse, K. W. (1988) The effect of age upon the affinity of microsomal monooxygenase enzymes for substrate in human liver. Age Ageing 17, 401–405. Imaoka, S., Fujita, S., and Funae, Y. (1991) Age-dependent expression of cytochromes P-450s in rat liver. Biochim. Biophys. Acta 1097, 187–192. Hunt, C. M., Westerkam, W. R., Stave, G. M., and Wilson, J. A. P. (1992) Hepatic cytochrome P-4503A (CYP3A) activity in the elderly. Mech. Ageing Dev. 64, 189–499. Hunt, C. M., Strater, S., and Stave, G. M. (1990) Effect of normal aging on the activity of human hepatic cytochrome P-450IIE1. Biochem. Pharmacol. 40, 1666–1669. Sotaniemi, E. A., Rautio, A., Lumme, P., Arvela, P., and Rautio, A. (1996) Age and CYP3A4 and CYP2A6 activities marked by the metabolism of lignocaine and coumarin in man. Therapie 51, 363–366. Sotaniemi, E. A., Arranto, A. J., Pelkonen, O., and Pasanen, M. (1997) Age and cytochrome P450-linked drug metabolism in humans: an analysis of 226 subjects with equal histopathologic conditions. Clin. Pharmacol. Ther. 61, 331–339. Burke, M. D. and Mayer, R. T. (1974) Ethoxyresorufin: Direct fluorometric assay of microsomal O-dealkylation which is preferentially induced by 3-methylcholanthrene. Drug Metab. Disp. 2, 583–588. Rodrigues, A. D. and Prough, R. A. (1991) Induction of cytochromes P450IA1 and P450IA2 and measurement of catalytic activities. Methods Enzymol. 206, 423–431. Ioannides, C. and Parke, D. V. (1975) Mechanism of induction of hepatic microsomal drug metabolising enzymes by a series of barbiturates. J. Pharm. Pharmacol. 27, 739–746.

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9 Assessing Age-Related Changes in Antioxidant Status The FRASC Assay for Total Antioxidant Power and Ascorbic Acid Concentration in Biological Fluids Iris F. F. Benzie and John J. Strain 1. Introduction There is accumulating evidence that oxidative damage to protein, lipid, carbohydrate and DNA is an important cause and/or effect of cellular and subcellular changes associated with disease, and is responsible for at least some of the physiological, but ultimately fatal, changes that accompany aging (1–8). Advancing age brings increasing risk of chronic degenerative disease including cancer, cardiovascular disease, cataracts, and dementia (1–6,8). Immune status declines, with consequent increased risk of infection and, owing to a combination of physical and socioeconomic factors, nutritional status is often poor in the elderly, increasing the likelihood of poor antioxidant status (9). Improved antioxidant status helps minimize oxidative damage, and this may delay or prevent pathological change (8–22). This suggests the possible utility of antioxidant-based dietary strategies for lowering risk of chronic, age-related disease (20–26). Vitamin C (ascorbic acid) and vitamin E (mainly α-tocopherol) are dietary derived antioxidants of major physiological importance (25,27–31), but many other exogenous and endogenous antioxidants contribute to the overall antioxidant status of the body (20,23,26,31,32). It is not yet possible to say that benefit to health is attributable to specific antioxidants at specific intake or plasma levels. Indeed it is likely that an optimal level of each antioxidant is required for maintenance of optimal health, that is, that an optimal “total” antioxidant status is desirable (23,27,33,34). In addition, the vitamin C to vitamin E ratio may be important, with risk of oxidative stress related From: Methods in Molecular Medicine, Vol. 38: Aging Methods and Protocols Edited by: Y. A. Barnett and C. R. Barnett © Humana Press Inc., Totowa, NJ

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disease increasing at ratios below 1.0 (34). What constitutes optimal antioxidant status is not yet clear, however, and further study of the role of antioxidant status, and of individual antioxidants and their interrelationships, in aging and age-related disease is needed. The ferric reducing (antioxidant) power (FRAP) assay,1 and its modified version, the simultaneous ferric reducing (antioxidant) power and ascorbic acid (FRASC) assay, is a technically simple, inexpensive, fast, sensitive, and robust biochemical test useful for the assessment of antioxidant status of biological fluids (35–37). FRASC can be performed using routinely available laboratory equipment, and permits the direct measurement of biological fluids such as blood plasma, cerebrospinal fluid, and urine. The antioxidant and ascorbic acid content of these fluids, and of extracts of various drugs and dietary agents, can be measured objectively and reproducibly using FRASC, allowing their potential for improving the antioxidant status of the body to be assessed and compared (37–42). The assay is also analytically sensitive and precise enough to assess postingestion response to dietary antioxidants. FRASC, therefore, offers a practical analytical tool to help assess diet-, disease-, or age-related changes in antioxidant status.

1.1. Rationale of the FRASC Assay for Total Antioxidant Power and Ascorbic Acid Concentration in Biological Fluids (36,37) In this assay, a ferric-tripyridyltriazine (FeIII–TPTZ) complex is reduced to its ferrous form, which is blue colored and absorbs light of 593 nm. The ferric to ferrous reaction is driven by the reductive action of electron donating antioxidants in the test sample, and the change in absorbance at 593 nm is directly proportional to the combined, or “total,” reducing (antioxidant) power of these antioxidants (35). In FRASC, ascorbic acid in the sample is selectively and specifically destroyed by ascorbate oxidase. The change in absorbance in this case is attributable to the remaining antioxidants, that is, the “total” less the contribution of ascorbic acid. The difference in antioxidant power between two paired samples, one treated with ascorbic oxidase and one untreated (and therefore still containing ascorbic acid) is equal to the contribution of ascorbic acid in the untreated sample (see Fig. 1). It is then a simple matter to calculate the molar concentration of ascorbic acid in the test sample, and to obtain three indices of antioxidant status: (1) the total antioxidant power, (2) the ascorbic acid concentration, and (3) the “non-ascorbic acid” antioxidant power of the sample. 2. Materials 1. 0.3 M Acetate buffer, pH 3.6; dissolve 3.1 g of sodium acetate trihydrate (Riedelde Haen, Germany) in approx 500 mL of distilled and deionized water; add 16 1

U.S. patent pending.

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Fig. 1. Measuring concept of FRASC. This figure shows the absorbance change owing to FeIII reduction by antioxidants in the sample. Calculation of FRAP value is by taking the 0–4-min change in absorbance at 593 nm for test sample (closed circles, 1) and relating it to the 0–4-min absorbance change for the FeII standard (closed triangles, 2), with a reagent blank correction (open triangles, 3) for both. Calculation of ascorbic acid results is by subtracting the 0–1-min absorbance reading of the ascorbate oxidasetreated test sample (open circles) from the matching water-treated sample (closed circles, 4); this signal is then related to that given by a standard solution of FeII (closed triangles) (or ascorbic acid, closed squares, 5) of appropriate concentration. (Reproduced with permission from Benzie, I. F. F. and Strain, J. J. (1997) Redox Report 3, 233–238.)

2.

3. 4.

5.

mL of glacial acetic acid (BDH Laboratory Supplies, England), and make up to a final volume of 1 L with distilled and deionized water. This solution can be stored at room temperature for up to 1 mo. 0.01 M TPTZ (2,4,6 tripyridyl-s-triazine, Fluka Chemicals, Switzerland) in 0.04 M HCl (BDH). This solution can be stored at room temperature for up to 2 wk (see Note 1). 0.02 M FeCl3.6H2O (BDH). This solution can be stored at room temperature for up to 2 wk. 4000 U/L of ascorbate oxidase (EC 1.10.3.3) (Sigma Chemical, St. Louis, MO, USA) in distilled water. Aliquots of this solution should be stored at –70°C and thawed when required. To prepare working FRASC reagent, mix 20.0 mL of 0.3 M acetate buffer, pH 3.6, 2.0 mL of 0.01 M TPTZ solution in 0.04 M HCl; and 2.0 mL of 0.02 M FeCl3·6H2O solution just before use.

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6. For calibration, standard solutions of FeSO4·7H2O (Riedel de Haen, Germany) made up in water to a known concentration; for example, 100 µM, 500 µM, and 1000 µM are recommended (see Note 2). Reaction of FeII represents a one-electron exchange reaction and is taken as unity, that is, the blank corrected signal given by 1000 µM solution of FeII is equivalent to a ferric reducing/antioxidant power (FRAP) value of 1000 µM. As ascorbic acid has a stoichiometric factor of 2.0 in this assay (35–37), reaction of ascorbic acid gives a change in absorbance double that of an equivalent molar concentration of FeII , that is, a FeII standard of 100 µM is equivalent to 50 µM ascorbic acid, and a 100 µM solution of ascorbic acid has a FRAP value of 200 µM. Freshly prepared aqueous ascorbic acid solutions (ascorbic acid extra pure crystals, Sigma Chemical, St. Louis, MO, USA) can also be used as calibrators (36,37,42) (see Note 3). Refer to Note 4 for guidelines on quality control samples and expected linearity and precision.

3. Methods 1. Samples: Serum, plasma, urine, saliva, tears, other biological fluids, and aqueous and ethanolic extracts of drugs and foodstuffs can be used directly in FRASC. However, as some antioxidants are unstable, samples should be kept chilled and in the dark until testing, and should be tested with as little delay as possible. Hemolyzed plasma or serum samples should be avoided. Heparinized plasma is preferable to EDTA plasma and serum for FRAP and FRASC measurements, as ascorbic acid is more stable in heparinized plasma (39). 2. To measure the total antioxidant power, as FRAP, and ascorbic acid in one test (FRASC), ascorbic acid in one of a matching pair of sample aliquots is destroyed by the addition of ascorbate oxidase. Ascorbic acid reacts very quickly with the working reagent, and the 0–1-min reaction time window is used for calculation of ascorbic acid results; the 0–4-min window is used for calculation of “total” antioxidant power (FRAP) results, that is, absorbance readings are taken at 0, 1, and 4 min after reagent/sample mixing (37°C incubation). 3. To prepare samples: a. Add 40 µL of a 4000 U/L solution of ascorbic oxidase to 100 µL of test sample. b. Add 40 µL of distilled water to the paired 100-µL sample aliquot. c. Calibrators and QC samples are treated similarly in pairs (see Note 5). This predilution of samples, calibrators and QC samples can be performed in the analyzer sample cups. 4. The paired ascorbate oxidase diluted (“+ao”) and water-diluted (“–ao”) samples are then immediately loaded onto the analyzer for automated measurement (see Note 6).

3.2. Data Collection and Calculation of Results 1. The FRASC assay can be performed using any type of automated analyzer that permits blank corrected readings at 593 nm to be taken at selected intervals after

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Table 1 Cobas Fara Test Program for FRASC Assay Measurement code Reaction mode Reagent blank Wavelength Temperature R1 M1 Sample volume Diluent name Volume Readings First Number Interval Reaction direction Number of steps Calculation First Last

Abs R1-I-S-A reag/dil 593 nm 37°C 300 µL 1.0 s 10 µL H2O 30 µL 0.5 s 17 15 s Increase 1 Endpoint M1 17 (i.e., 0–4 min) for FRAP; reading 5 (i.e., 0–1 min reading) is retrieved for calculation of ascorbic acid

Modified from Benzie, I. F. F. and Strain, J. J. (1997) Redox Report 3, 233–238.

2.

3. 4. 5.

sample-reagent mixing. In our laboratories the Cobas Fara centrifugal analyzer (Roche Diagnostics Ltd., Basel, Switzerland) is used, and the user-defined test program is presented in Table 1. The 0–4-min reaction time window is used for data capture for the FRAP value. The absorbance change is translated into a FRAP value by relating the change of absorbance at 593 nm of test sample to that of a standard solution of known FRAP value, for example, 1000 µM FeII, as described in Eq. 1. To obtain ascorbic acid results, the 0–1-min absorbance at 593 nm readings are retrieved, and calculation of results is performed as described in Eqs. 2 and 3. The nonascorbic acid antioxidant power (see Note 7) is calculated according to Eq. 4. Calculation of results is as follows: Using the water-diluted samples, the FRAP (µM) value = [FRAP]std (µM) × 0–4 minute A593 nm test sample / 0–4 minute A593 nm standard (see Note 6).

(1)

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Table 2 FRAP Values and Ascorbic Acid Concentrations (Mean; Median; SD; µmol/L), Using FRASC, of Fresh Fasting EDTA Plasma from Healthy Subjects Age (years) FRAP Ascorbic acid

All (n = 130)

Men (n = 66)

Women (n = 64)

43; 43; 16.4 1018; 1004; 198 51; 48; 17.9

42; 42; 16.3 1086; 1077; 189 49; 48; 13.8

43; 44; 16.6 948; 927; 183 52; 50; 21.3

Reproduced with permission from Benzie, I. F. F. and Strain, J. J. (1997) Redox Report 3, 233–238.

Using the paired water (–ao) and ascorbate oxidase diluted (+ao) samples, the ascorbic acid concentration is calculated as follows: 0–1 min ascorbic acid related change in A593 nm = (0–1 min A593 nm sample –ao) – (0–1 min A593 nm sample +ao)

(2)

ascorbic acid concentration (µM) = [ascorbic acid] std (µM) × 0–1 min ascorbic acid related A593 nm of test sample / 0–1 min ascorbic acid related A593 nm of standard

(3)

(see Note 8). nonascorbic acid antioxidant power = FRAP value (µM) – 2 × ascorbic acid concentration (µM)

(4)

Table 2 gives typical values obtained on fasting plasma samples from healthy adults.

4. Notes 1. The TPTZ powder, as purchased, is normally white. However, in some bottles, when opened, the contents have been found to be gray or yellow in color. The reason for this coloration is not clear, but it does not appear to affect results. The working FRASC reagent should be a pale yellow/orange color. Any visible blue color indicates contamination by either ferrous iron or a reducing agent. Visible blue color in the working reagent will give a high blank reading and decrease sensitivity: do not use. 2. Do not attempt to use FeII standards >1500 µM, as there will be precipitation of iron salts. Aqueous ferrous sulfate solutions of up to 1500 µM appear to be stable for at least 1 mo at 4°C. These solutions should be clear and colorless. 3. It is important that ALL samples, calibrators, and QC samples be treated identically and in matching ascorbate oxidase diluted and water diluted pairs. The same absorbance reading should be obtained for the paired FeII solutions, that is, the

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4.

5. 6.

7.

8.

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addition of ascorbate oxidase should not result in any difference in the absorbance given as there is no ascorbic acid to destroy in these solutions. Guidelines on quality control samples and expected performance: For ease of use and reliability, aqueous ascorbic acid solutions at 100, 250, 500, and 1000 µM (equivalent to 200, 500, 1000, and 2000 µM FRAP, prepared fresh daily) and aged QC serum freshly spiked with ascorbic acid are recommended as quality control samples. These should be run in parallel with test samples to actively monitor the performance of the test and to ensure comparability with previous results. Expected precision and sensitivity of the FRASC assay: Precision is high in FRASC: typical within- and between- run CVs obtained in our laboratories are, respectively, 85% for point mutations), a number of different methods have been developed to accurately measure the level of mtDNA heteroplasmy, thus providing more clues about the molecular basis of their pathogenesis. These include PCRsingle-strand conformation polymorphism (PCR-SSCP) analysis, multiple clonal analysis, allele-specific oligonucleotide hybridization, and more recently a fluorescence-based primer extension method (11–14). However, for many laboratories, the method of choice remains PCR-restriction fragment length polymorphism (RFLP) analysis, especially as this is readily applicable to the study of mtDNA heteroplasmy in single cells. Essentially, the region of mtDNA containing the point mutation under investigation is amplified by PCR, radiolabeled, and digested by a restriction endonuclease that can discriminate between wild-type and mutant mtDNA molecules at the site of the point mutation. Digestion products are electrophoresed through either agarose or nondenaturing polyacrylamide gels and the level of heteroplasmy calculated by determining the incorporation of radiolabel into each of the restriction products. The formation of heteroduplex molecules during the PCR reaction can make quantitation difficult because these are not digested by restriction endonucleases. However, the addition of radiolabel to the last cycle of amplification (hence “last hot cycle PCR”) avoids the detection of these heteroduplexes, allowing an accurate determination of heteroplasmy (15,16). We describe in the following section the methodology associated with determining the level of mtDNA heteroplasmy in both tissue homogenates and single cells. As an example, we describe the strategy used to investigate a patient previously described as heteroplasmic for a T10010C transition in the tRNAGly gene (17). 2. Materials

2.1. Automated Sequencing of mtDNA 1. 2 mM (10×) dNTP mix for PCR. Store at –20°C 2. 10× GeneAmp PCR buffer: 100 mM Tris-HCl, pH 8.3, 500 mM KCl, 15 mM MgCl2, 0.01% (w/v) gelatin (Perkin–Elmer). Store at –20°C. 3. AmpliTaq Gold™ DNA polymerase (5 U/µL) (Perkin–Elmer). Store at –20°C. 4. Template DNA: Dilute to 200 ng/µL for PCR amplification. 5. Oligonucleotide primers for PCR: Owing to the length of read that is now possible using automated DNA sequencers, the mitochondrial genome is amplified in fragments of between 650 and 750 basepairs using a set of forward (L) and reverse (H) primer pairs (e.g., 01F and 01R) that have been designed to anneal at 58°C. Both sets of primers have 18 bases of M13 sequence at their 5' end, allowing the products to be cycle sequenced. The M13 sequences used are: forward (L) primers are prefixed by 5' TGTAAAACGACGGCCAGT 3'; reverse (H) primers

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7. 8. 9.

10. 11. 12. 13.

14. 15. 16. 17.

Taylor et al. are prefixed by 5' CAGGAAACAGCTATGACC 3' (see Tables 1 and 2). Owing to the length of these PCR primers (35–40 basepairs), we recommend that they are hplc purified following synthesis. Stocks for PCR amplification (20 µM) are stored at –20°C (see Note 2). DNA thermal cycler: This is required for the initial PCR amplifications and the subsequent cycle sequencing. To achieve a high throughput of samples, a thermal cycler capable of running 96 samples is desirable. We use the GeneAmp® PCR System 9700 (Perkin–Elmer) which has a heated lid facility, thereby negating the need for mineral oil overlay for either the PCR amplification or cycle sequencing reactions. Sterile water. 0.2 mL—MicroAmp® Reaction tubes (Perkin–Elmer). Horizontal gel electrophoresis equipment for running 1% agarose gels; 1× TAE (40 mM Tris-acetate, 1 mM EDTA, pH 8.0) running buffer containing ethidium bromide. UV transilluminator. QIAquick PCR Purification Columns (Qiagen). Microfuge capable of holding 1.5-mL Eppendorf tubes. Dye primer cycle sequencing ready reaction kits, –21 M13 and M13Rev (Perkin–Elmer). Because both of the primers used in the PCR amplification of the fragment of interest have M13 tails, the product can be sequenced in both (forward and reverse) directions. Store these kits at –20°C. Bucket microfuge capable of spinning a 96-well PCR tray. Ethanol, 95% (v/v). Glycogen, 0.3 mg/mL solution. Store at –20°C. Sample buffer: A 5:1 (v/v) mixture of deionized formamide and 25 mM EDTA, pH 8.0, containing blue dextran (50 mg/mL).

2.2. Last Hot Cycle PCR 1. Template DNA: Although initial studies are likely to be performed on total DNA extracted from a tissue homogenate, this technique is perfectly suited to the investigation of mtDNA mutations in single cells. Methods describing the isolation of DNA from single cells are found in the preceding chapter. 2. PCR amplification: The following stock solutions are required: 2 mM (10×) dNTPs (Boehringer Mannheim), GeneAmp® PCR buffer containing 100 mM Tris-HCl, pH 8.3; 500 mM KCl; and 15 mM MgCl2 (Perkin–Elmer), AmpliTaq® DNA polymerase (Perkin–Elmer). 3. Oligonucleotide primers: These will amplify a region of the mitochondrial genome containing the mutation of interest. In this case, a 350-basepair fragment is amplified using L9695 (nt 9695–9717) and H10044 (nt 10044–10022). Stock solutions (20 µM) are stored at –20°C. 4. Sterile water. 5. PCR tubes: 0.5-mL Thermotubes (Applied Biosystems) are recommended. 6. Ice: All PCR reactions are set up on ice.

Mitochondrial DNA Mutations 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21.

22. 23. 24.

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PCR thermal cycler. Horizontal gel electrophoresis equipment. 1% Agarose gels containing ethidium bromide, 1× TAE running buffer. UV transilluminator [α-32P] dCTP (3000 Ci/mmol) (Amersham Life Science Products). Phenol (Molecular Biology grade). Chloroform:isoamyl alcohol (24:1 [v/v]). 7.5 M Ammonium acetate, sterile. Ethanol, 100%. Ethanol, 70% (v/v). Cerenkov counter. Appropriate restriction enzyme supplied with 10× reaction buffer: For the investigation of the T10010C mutation, the restriction endonuclease RsaI (Boehringer Mannheim) is required. Heat block with variable temperature setting. Vertical electrophoresis system: We regularly use a 16 cm unit (SE 600) manufactured by Hoefer (Pharmacia Biotech). 5% Nondenaturing polyacrylamide gel: This is made using a 30% polyacrylamide stock solution (29:1 acrylamide/bisacrylamide [w/w]) and contains 1× TBE (45 mM Tris-borate, 1 mM EDTA, pH 8.0) as the buffering component. 1× TBE (45 mM Tris-borate, 1 mM EDTA, pH 8.0) running buffer. Gel drying equipment (Bio-Rad Laboratories). Phosphorimage cassette and imaging system, including ImageQuant software (Molecular Dynamics).

3. Methods

3.1. Automated Sequencing of mtDNA 1. For each PCR amplification, prepare the following reaction at room temperature: 35.75 µL Sterile water 5 µL 10× PCR reaction buffer 5 µL 10× (2 mM) dNTPs 1.5 µL 20 µM forward primer 1.5 µL 20 µM reverse primer 1 µL Template DNA (200 ng/µL stock) 0.25 µL AmpliTaq Gold™ 2. Centrifuge briefly and place in PCR thermal cycler. 3. Perform 30 cycles of amplification as follows: Initial denaturation 94°C 12 min 30 cycles 94°C 30 s 58°C 30 s 72°C 30 s Final extension 72°C 8 min

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4. Electrophorese samples (5 µL) through a 1% agarose gel containing 0.4 µg/mL of ethidium bromide for about 1 h, and visualize by UV transillumination. 5. Purify the remaining sample using a QIaquick PCR purification column according to the manufacturer’s instructions. At the last step, elute the DNA into a volume of water to give a DNA concentration appropriate for the length of fragment to be sequenced (e.g., a sample of 500 basepairs will need to be at a concentration of about 7 ng/µL). Once purified, these samples can be stored at –20°C prior to cycle sequencing. 6. When sequencing, thaw the dye primer cycle sequencing ready reaction mixes slowly on ice, and vortex to mix well. 7. Combine the following into four separate PCR tubes: Reagent Reaction: A (µL) C (µL) G (µL) T (µL) Ready Reaction Premix 4 4 8 8 PCR product 1 1 2 2 8. Vortex briefly to mix, and centrifuge to bring the sample to the bottom of the tube. 9. Place the tubes in a thermal cycler, set the reaction volume to “10 µL,” and cycle sequence using the following linked cycles: 15 Cycles

linked to 15 Cycles

10. 11. 12. 13. 14. 15.

16.

96°C 55°C 70°C

10 s 5s 60 s

96°C 10 s 70°C 60 s Aliquot 80 µL of 95% ethanol and 5 µL of glycogen into a fresh sterile PCR tube. Add the four extension reactions (A, C, G, and T) into the ethanol/glycogen mix, cover the tubes with aluminum foil, and vortex-mix briefly. Place the tubes on ice and leave for 15 min to precipitate the extension products. Centrifuge in the bucket centrifuge for 15 min (1800gav). Discard the foil and decant the supernatant by inverting the tubes over a paper towel and centrifuging for a further minute at 100gav. Air-dry the resulting pellets at room temperature for 5 min and replace on ice. These can be stored at –20°C for several months prior to resuspension and electrophoresis. Just prior to electrophoresis, the pellet is dissolved in 3 µL of sample buffer (see Subheading 2.1.). The DNA sample is heated at 90°C for 2 min, placed on ice immediately and loaded onto a 6% polyacrylamide gel (29:1 [w/w] acrylamide/ bisacrylamide, Bio-Rad Laboratories) containing 8 M urea that has been preelectrophoresed for 15 min in 1× TBE buffer. Samples are electrophoresed at 39 W for 14 h, and sequence data collected on an ABI Model 373A automated DNA sequencer (Applied Biosystems). Factura and Navigator sequence analysis software (Perkin–Elmer, Applied Biosystems Division) are used to compare and align sequence files (see Note 3).

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3.2. Last Hot Cycle PCR 1. For each PCR amplification, prepare the following reaction: 35.75 µL 5 µL 5 µL 1.5 µL 1.5 µL 1 µL 0.25 µL

Sterile water 10× PCR reaction buffer 10× (2 mM) dNTPs 20 µM forward (L9695) primer 20 µM reverse (H10044) primer Template DNA (200 ng/µL stock)* AmpliTaq® DNA polymerase

*If template DNA is from a single cell lysis, reduce the volume of water accordingly. A master mix of these components without the template DNA can be made if multiple samples are being investigated. Overlay with mineral oil if required. 2. Centrifuge briefly and place in PCR thermal cycler 3. Perform 30 cycles of amplification as follows: Initial denaturation 30 Cycles

Final extension

94°C 94°C 56°C 72°C 72°C

8 min 1 min 1 min 1 min 8 min

4. Electrophorese samples (5 µL) through a 1% agarose gel containing 0.4 µg/mL of ethidium bromide and visualize by UV transillumination. 5. If the amplification is successful, add another 1.5 µL of each primer, 0.25 µL AmpliTaq® DNA polymerase, and 0.5 µL of [α-32P] dCTP (3000 Ci/mmol) to each reaction, and perform the following cycle: Denaturation Annealing Final extension

94°C 56°C 72°C

8 min 1 min 12 min

6. Extract the products with 50 µL of phenol, followed by 50 µL of phenol:chloroform:isoamyl alcohol (25:24:1). 7. Precipitate the labeled products by the addition of 25 µL of 7.5 M ammonium acetate (1⁄2 volume) and 100 µL of 100% ethanol (2 vol) and place at –80°C for 1 h. 8. Centrifuge the precipitated products, wash with 70% ethanol, and air-dry. 9. Count the pellets using the Cerenkov counter, and resuspend in sterile water so that equal amounts (2000–8000 cpm) are digested. 10. Set up the restriction digest in a final volume of 20 µL using the appropriate restriction endonuclease (5–10 U) and 10× reaction buffer as recommended by the manufacturer. 11. Separate the digested products through a 5% nondenaturing polyacrylamide gel, dry the gel, and expose to a PhosphorImage cassette (see Note 4). 12. Using the available software, determine the level of heteroplasmy in the sample by calculating the amount of radiolabel in each restriction fragment, ensuring

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Fig. 1. PCR-RFLP analysis of mtDNA heteroplasmy in a patient with a pathogenic T10010C mutation. U, uncut PCR product; C, control subject; M, skeletal muscle showing a high level of mutant mtDNA; B, blood that has a much lower level of heteroplasmy. In the presence of the mutation, the 297-basepair fragment remains uncut.

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that the digestion has gone to completion. For the T10010C mutation analysis, there are two RsaI recognition sites in the wild type product, which generate fragments of 263, 53, and 34 basepairs. In the presence of the mutation, a site is lost, leaving two fragments of 297 and 53 basepairs. For quantitation, the 263-basepair fragment is normalized to the 297-basepair fragment for deoxycytosine content, and the level of heteroplasmy calculated as a percentage of the amount of radiolabel in the 263-basepair fragment relative to the combined amount in the 263 and 297-basepair fragments (Fig. 1) (see also Notes 5–7).

4. Notes 1. Although dye primer cycle sequencing typically provides an even peak height making it easier to detect heteroplasmic base changes, peak height is not directly proportional to the level of mtDNA heteroplasmy within a sample. Consequently, proportions of mutant or wild-type mtDNA below 30% are unlikely to be detected. If a novel change from the Cambridge sequence (8) is detected that does not appear to be a recognized polymorphism, last hot cycle PCR-RFLP analysis should be performed to exclude or confirm the presence of heteroplasmy and its level within the DNA sample. 2. As previously mentioned, we recommend that both PCR primers are synthesized with the universal sequencing primer sequences at their 5' ends, thus allowing each PCR product to be sequenced bidirectionally and any base substitutions confirmed. Moreover, there are occasionally small lengths of sequence (usually less than 5 basepairs) that cannot be read in one direction, presumably due to the nature of the DNA secondary structure. Sequencing of these samples in the opposite direction resolves this problem. 3. The complementary DNA strands of the mitochondrial genome have an asymmetric distribution of G’s and C’s, generating a heavy purine-rich H-strand, and a light pyrimidine-rich L-strand. Sequence data from the H-strand typically show higher levels of background noise and poorer base-calling than that obtained from sequencing the L-strand. 4. A sample of the undigested, labeled PCR product must always be run on the gel to ensure that the restriction digest is 100% efficient. 5. The most critical part of this method is the design of the RFLP to analyze the mutation. Ideally, the PCR-generated fragment should have two restriction sites for the enzyme used, one of which is unique to the mutation. This can be either gain or loss of a site. If the putative mutation does not create or destroy a naturally occurring restriction site, it is possible to engineer a site that is specific for the mutation into the PCR fragment using a mismatch primer (7,18). 6. Instead of performing a last hot cycle, the use of radioactivity can be avoided by the addition of fluorescent-labeled deoxyuridinetriphosphates (dUTPs) to the final cycle of PCR, separating the restriction products by nondenaturing polyacrylamide gel electrophoresis (PAGE) (ABI Model 373A automated DNA sequencer [Applied Biosystems]), and quantitating the level of heteroplasmy using Genescan software (Applied Biosystems) (19).

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8. Quantitating the level of heteroplasmy in single cells (e.g., individual muscle fibers) will confirm whether high levels of the mutation above the required threshold for disease expression precipitate an observable biochemical defect. In this case, single-fiber PCR analysis will reveal significantly higher levels of mutant mtDNA in COX-deficient fibers than in COX-positive fibers. The protocols describing the isolation of template DNA from single cells are described in detail in the preceding chapter.

Acknowledgments The financial support of the Muscular Dystrophy Group of Great Britain, the Medical Research Council, and the Wellcome Trust is gratefully acknowledged. References 1. Chinnery, P. F. and Turnbull, D. M. (1997) Clinical features, investigation, and management of patients with defects of mitochondrial DNA. J. Neurol. Neurosurg. Psychiatr. 63, 559–563. 2. Shoffner, J. M. (1996) Maternal inheritance and the evaluation of oxidative phosphorylation diseases. Lancet 348, 1283–1288. 3. Schon, E. A., Bonilla, E., and DiMauro, S. (1997) Mitochondrial DNA mutations and pathogenesis. J. Bioenerg. Biomembr. 29, 131–149. 4. Taylor, R. W., Chinnery, P. F., Haldane, F., Morris, A. A. M., Bindoff, L. A., Wilson, J., and Turnbull, D. M. (1996) MELAS associated with a mutation in the valine transfer RNA gene of mitochondrial DNA. Ann. Neurol. 40, 459–462. 5. Chinnery, P. F., Johnson, M. A., Taylor, R. W., Lightowlers, R. N., and Turnbull, D. M. (1997) A novel mitochondrial tRNA phenylalanine mutation presenting with acute rhabdomyolysis. Ann. Neurol. 41, 408–410. 6. Chinnery, P. F., Johnson, M. A., Taylor, R. W., Durward, W. F., and Turnbull, D. M. (1997) A novel mitochondrial tRNA isoleucine mutation causing chronic progressive external ophthalmoplegia. Neurology 49, 1166–1168. 7. Taylor, R. W., Chinnery, P. F., Bates, M. J. D., Jackson, M. J., Johnson, M. A., Andrews, R. M., and Turnbull, D. M. (1998) A novel mitochondrial DNA point mutation in the tRNAIle gene: studies in a patient presenting with chronic progressive external ophthalmoplegia and multiple sclerosis. Biochem. Biophys. Res. Commun. 243, 47–51. 8. Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., Eperon, I. C., Nierlich, D. P., Roe, B. A., Sanger, F., Schreier, P. H., Smith, A. J., Staden, R., and Young, I. G. (1981) Sequence and organisation of the human mitochondrial genome. Nature 290, 457–465. 9. Lightowlers, R. N., Chinnery, P. F., Turnbull, D. M., and Howell, N. (1997) Mammalian mitochondrial genetics: heredity, heteroplasmy and disease. Trends Genet. 13, 450–455. 10. Tanaka, M., Hayakawa, M., and Ozawa, T. (1996) Automated sequencing of Mitochondrial DNA. Methods Enzymol. 264, 407–421.

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11. Tanno, Y., Yoneda, M., Tanaka, K., Tanaka, H., Yamazaki, M., Nishizawa, M., Wakabayashi, K., Ohama, E., and Tsuji, S. (1995) Quantitation of heteroplasmy of mitochondrial tRNALeu(UUR) gene using PCR-SSCP. Muscle Nerve 18, 1390–1397. 12. Howell, N., Halvorson, S., Kubacka, I., Mccullough, D. A., Bindoff, L. A., and Turnbull, D. M. (1992) Mitochondrial gene segregation in mammals: is the bottleneck always narrow? Hum. Genet. 90, 117–120. 13. Ghosh, S. S., Fahy, E., Bodis-Wollner, I., Sherman, J., and Howell, N. (1996) Longitudinal study of a heteroplasmic 3460 Leber hereditary optic neuropathy family by multiplexed primer-extension analysis and nucleotide sequencing. Am. J. Hum. Genet. 58, 325–334. 14. Fahy, E., Nazarbaghi, R., Zomorrodi, M., Herrnstadt, C., Parker, W. D., Davis, R. E., and Ghosh, S. S. (1997) Multiple fluorescence-based primer extension method for quantitative mutation analysis of mitochondrial DNA and its diagnostic application to Alzheimer’s disease. Nucleic Acids Res. 25, 3102–3109. 15. Tanno, Y., Yoneda, M., Nonaka, I., Tanaka, K., Miyatake, T., and Tsuji, S. (1991) Quantitation of mitochondrial DNA carrying tRNALys mutation in MERRF patients. Biochem. Biophys. Res. Commun. 179, 880–885. 16. Moraes, C. T., Ricci, E., Bonilla, E., DiMauro, S., and Schon, E. A. (1992) The mitochondrial tRNALeu(UUR) mutation in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS): genetic, biochemical, and morphological correlations in skeletal muscle. Am. J. Hum. Genet. 50, 934–939. 17. Bidooki, S. K., Johnson, M. A., Chrzanowska-Lightowlers, Z., Bindoff, L. A., and Lightowlers, R. N. (1997) Intracellular mitochondrial triplasmy in a patient with two heteroplasmic base changes. Am. J. Hum. Genet. 60, 1430–1438. 18. Yoneda, M., Tanno, Y., Tsuji, S., and Attardi, G. (1996) Detection and quantification of point mutations in mitochondrial DNA by PCR. Methods Enzymol. 264, 432–441. 19. Chalmers, R. M., Lamont, P. J., Nelson, I., Ellison, D. W., Thomas, N. H., Harding, A. E., and Hammans, S. R. (1997) A mitochondrial DNA tRNAVal point mutation associated with adult-onset Leigh syndrome. Neurology 49, 589–592.

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21 Assessment of T-Cell Function in the Aged T-Cell Proliferative and T-Cell Adherence Assays Ian Beckman 1. Introduction Certain immunological activities, particularly cell-mediated immunity, decline with advancing age. Gaining insight into the underlying mechanism(s) is complicated by the fact that human T-cells comprise several functionally and phenotypically distinct populations and subpopulations. Proliferative studies designed to identify differences between old and young subjects that are based entirely on unfractionated peripheral blood lymphocytes (PBLs) or pan T-cell responses run the risk, therefore, of missing subtle but perhaps crucial changes in a particular T-cell type. Although it is outside the scope of this chapter to describe T-cell purification procedures, it is important to emphasize several key considerations before venturing down this road. First, choose several different procedures that operate sequentially rather than one method repeated several times over. For example, my laboratory employs simple nylon wool columns to obtain an initial 90–95% purity (nylon wool-adherent cells also make an excellent source of blood monocytes if required in subsequent cell mixing experiments). The partially purified T-cells are then further depleted of residual monocytes by layering over plastic Petri dishes before being divided into various subpopulations (e.g., CD4+CD45RO+ or CD8+CD28+) either by negative selection, using two rounds of antibody panning with an appropriate cocktail of monoclonal antibodies (MAbs), or by positive selection, via flow microfluorimetry sorting (1,2). Magnetic beads are, of course, another option.

From: Methods in Molecular Medicine, Vol. 38: Aging Methods and Protocols Edited by: Y. A. Barnett and C. R. Barnett © Humana Press Inc., Totowa, NJ

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Whatever the mechanism employed, it is important that the final T-cell fraction is free of B cells (and other T-cell subpopulations if desired), monocytes, and natural killer (NK) cells. For example, “young” T-cells in particular can respond well to a single mitogenic signal such as phytohemagglutinin (PHA) with < 1% contaminating monocytes. NK cells, on the other hand, are often overrepresented in the circulation of aged individuals and because NK cells can be purified along with T-cells, they not only may skew the final “T” cell count but also are a potent source of interferon-γ when activated. Some of the advantages of working with purified populations of T-cells are that functional changes can be related to a specific cell type(s), and most importantly, when comparing young and aged subjects, the actual number of T-cells (and indeed, accessory cells) seeded into a given well can be controlled in all experiments. The disadvantages of multiple purification procedures are low cell yields (despite relatively large volumes of blood) and increased risk of contamination. It is also important that the purification process per se does not preactivate the cells. For example, avoid using strongly mitogenic MAbs or sheep red blood cells (SRBCs) during positive selection. Although T-cell binding to SRBCs (E-rosettes) is a well-known means of enriching for T-cells, very low numbers of SRBCs provide sufficient stimulus to activate purified T-cells in the presence of PHA. Human peripheral T-cells are normally quiescent. They can be activated to secrete interleukin-2 (IL-2) and proliferate by a variety of stimuli. In general, complete T-cell activation requires two signals. The first is delivered through the T-cell receptor (TCR)–CD3 complex by small antigenic peptides in association with HLA molecules. This is best achieved in the laboratory using reagents such as superantigens or MAbs directed against TCR or CD3. The first signal can also be triggered by ligands that bind the surface receptor CD2. Second signals follow engagement of ligands on the surface of antigen presenting cells (APCs) with T-cell surface receptors (TCRs) such as CD28. However, a variety of molecules and in vitro systems can bypass the need for APCs or other accessory cells (ACs), making it possible to dissect specific T-cell–AC interactions and identify actual sites of dysfunction. Two potent pan T-cell mitogens, namely MAb OKT3 (anti-CD3) and the superantigen, staphylococcal enterotoxin B (SEB), have an obligatory requirement for ACs. However, OKT3-induced activation can be achieved in the absence of ACs either using wells precoated with anti-mouse IgG, which promotes crosslinking of the TCR–CD3 complex, or using plate-bound OKT3 and fibronectin. The latter combination is a particularly strong T-cell stimulus. Alternatively, plate-bound 64.1 (an IgM anti-CD3 MAb) alone can induce proliferation in highly purified T-cells.

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With respect to CD2-induced stimulation, mitogens such as PHA and several anti-CD2 MAbs that trigger the CD2 receptor are powerful activators. Interestingly, CD2-induced activation is markedly diminished in the elderly and because PHA is relatively nonspecific, CD2 activation is probably best observed using the anti-CD2 MAb pair T112 and T113. Together they are strongly mitogenic for T-cells in the presence of ACs. More importantly, when used with purified T-cells at relatively low concentrations, this pair of MAbs provides a potent signal 1 without causing IL-2 secretion. Full activation is achieved, however, when used in combination with any one of a number of different costimulatory factors or cells. To this end, various cofactors can easily be tested for their ability to provide an effective signal 2. Proliferation or activation can be determined in a number of ways. Clearly, the choice of the primary readout system is best governed by the type of information required. Other factors, including the availability of technical expertise, the sensitivity and specificity of the assay, cost, and time are also important considerations. The simplest approach is to measure tritiated thymidine incorporation. Probably the best marker of T-cell activation, however, is the production of IL-2 and other T-cell-derived cytokines. Resting unstimulated human T-cells remain transcriptionally silent even after several days of in vitro culture. Cytokines can be detected at the mRNA level by reverse transcriptase-polymerase chain reaction (RT-PCR), in situ hybridization using labeled probes, or by in situ PCR. The RT-PCR method (described in detail below) is relatively simple to perform; it is also fast, sensitive and semiquantitative (if required). Moreover, it allows the expression of a large range of activation-associated genes to be examined simultaneously using very small cell numbers, for example, cytokines and their receptors, other immune function genes (e.g., HLA DR, LFA–1, transferrin receptor, adhesion receptors, CD80, CD86, CD40), cellcycle-associated genes, proto-oncogenes etc. At the protein level, cytokines are readily detected by enzyme-linked immunosorbent assay (ELISA) (using cell supernatants) and in situ immunohistochemistry. The latter technique is very impressive (particularly when coupled to double or triple labeling and flow cytometry allowing the accurate enumeration of specific cell types, e.g., IL-4 producing CD4+CD45RO+ T-cells) but the trick to getting it to work consistently is obtaining the right MAbs (e.g., see refs. 3–5). It is definitely a procedure worthwhile pursuing in the aging area. Immunophenotyping and cell-cycle analysis using BrdU or propidium iodide could also be used to complement the above readouts. Wherever possible, utilize a number of different detection systems when comparing T-cell proliferation in aged and young subjects. Finally on the subject of read-out, if “old” T-cells do not appear to respond to a particular stimulus, check if they have actually apoptosed.

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T-cell adherence to vascular endothelium is, I believe, an important issue in immunogerontological studies. To egress the circulation and enter tissues, T-cells must interact with, and adhere to, endothelial cells. Changes in T-cell– endothelium interactions may well have implications for atherogenesis and the observed age-related decline in cell-mediated immunity, particularly delayed type hypersensitivity (DTH). For example, activated “old” T-cells may display an increased propensity to “stick” to endothelial cells (EC) and facilitate inflammation, or conversely, they may exhibit a diminished ability to bind and transmigrate through the endothelium. To this end, I have included in this chapter a simple assay that provides a measure of the capacity of various T-cells to bind human umbilical vein endothelial cells. 2. Materials 1. Complete culture medium (CCM); RPMI-1640 (Flow Labs) supplemented with 10% heat-inactivated fetal calf serum (FCS), 2 mM glutamine (added just prior to culture) and penicillin/streptomycin (10,000 U/mL) (see Note 1). 2. Sterile flat, 96-well microtiter (Linbro) and 24-well tissue culture (Costar) plates with lids (see Note 2). 3. Phosphate buffered saline (PBS). 4. 50 mM Tris-HCl, pH 9.5. 5. Tritiated thymidine ([3H]-TdR) (Amersham). 6. Exogenous activators: MAbs (OKT3 [anti-CD3] [from American Type Culture Collection], 64.1[anti-CD3] and 9.3 [anti-CD28] [from Bristol–Myers–Squibb, Seattle], T112 and T113 [anti-CD2] [from E. Reinherz, MIT, Boston MA, USA]); PHA (Sigma), phorbol 12-myristate 13-acetate (PMA) (Sigma), SEB (SEB) (from Sigma), fibronectin (Collaborative Research Incorporated, or Integrated Sciences), and various cytokines (Boehringer Mannheim and Genzyme). 7. Goat anti-mouse IgG (GAM) (Dakopatts). 8. Mitomycin C (Sigma). 9. Collagenase (Sigma). 10. Fluorescein-labeled (FITC-) goat or sheep anti-mouse IgG (Becton-Dickinson). 11. Vanyl ribonucleoside complexes (Gibco-BRL). 12. 123-basepair DNA ladder (Gibco-BRL). 13. Endothelial cell growth factor supplement (Integrated Sciences). 14. ECV304: Transformed immortal human endothelial cell line from umbilical cord (ATCC CRL-1998). 15. Trypsin/EDTA solution (1×) in PBS (Boehringer Mannheim).

3. Methods

3.1. T-Cell Activation Using Unfractionated PBL 1. Add 20 µL or 100 µL of MAb OKT3, at a single predefined optimal concentration (e.g., 50 ng/mL; note the final concentration in each well is 5 ng/mL) or use

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a range of concentrations, to triplicate wells of a 96- or 24-well plate, respectively (see Note 3). 2. Alternatively or in parallel experiments, add titrated amounts of SEB (a final concentration of 10 ng/mL is optimal in my laboratory) or MAbs T112 and T113 for anti-CD2 stimulation (as a guide we use a final dilution each of 1:500 ascites fluid). 3. Dispense 180 µL or 900 µL of PBL (prewashed twice in PBS and resuspended in CCM at 5.5 × 105/mL) to each 96- or 24-well plate, respectively, and incubate at 37°C in a 5% CO2 controlled atmosphere for 3–4 d. Try a range of cell concentrations ranging from 2 × 105/mL to 1 × 106/mL (see Note 4). 4. At the designated time(s), appropiate cultures are spiked with 0.5 µCi [3H]TdR for 4 h before harvesting the cells using a semiautomated sample harvester. The amount of [3H]TdR incorporation is measured in a beta scintillation counter (subtract the cpm recorded in negative control wells, i.e., wells containing nonactivated cells). Cells are also harvested from parallel cultures at identical times for other analyses, for example, RT-PCR, cell cycle, and/or immunohistochemical analysis (see Note 5). Remember to store the centrifuged culture supernatants at –80°C for cytokine ELISAs, if required. Alternatively, T-cell responses generated in one-way mixed lymphocyte reactions (MLR) using PBLs are analyzed by coculturing 1 × 105 cells (responder cells) from one individual with an equal number of irradiated (2000 rads) or mitomycin C-treated cells (stimulator cells) from one or more unrelated individuals in triplicate round-bottom 96-well plates in a final volume of 200 µL (see Note 6). Incubate for 7 d and spike with 0.5 µCi [3H]TdR per well for 16–18 h before harvesting as described previously.

3.2. T cell Activation Using Purified T Cells and AC 1. Cultures are basically set up as described previously except purified T-cells are employed (see Note 7); however, in these experiments the proportion of ACs or monocytes to T-cells is controlled for each subject. 2. We usually add 10% irradiated (2000 rads) AC-enriched cells (i.e., nylon wooladherent and plastic-adherent peripheral blood mononuclear cells) to each well containing 1 × 105 or 1 × 106 T-cells (see Note 8).

Again, it is often informative to try a range of AC concentrations (i.e., 5% to 30%). Furthermore, these experiments provide a degree of flexibility. That is, the ACs can be either (1) irradiated autologous ACs (2) irradiated heterologous ACs (a MLR is avoided here by culturing for 3 d) (3) transformed cell lines such as K562 or U937 (irradiated with 4000 rads to prevent outgrowth) (4) irradiated CHO transfectants expressing specific human ligands singularly or in combination, for example, HLA DR, CD80, CD86, LFA-3, CD40, Fas, etc., or (5), irradiated OKT3–hybridoma cells. Interestingly, the OKT3 expressing

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T3 hybridoma cells satisfy the dual requirements for full activation by providing both first and second signals. Judicious use of transfectants can help accurately dissect specific T-cell–AC interactions and thus provide real insight into potential age-related deficiencies.

3.3. AC-Independent T-Cell Activation 3.3.1 Anti-Mouse IgG-Coated Plates 1. Precoat wells with 750 ng/well goat anti-mouse IgG (GAM) prepared in 50 mM Tris-HCl, pH 9.5. 2. Leave overnight at 4°C, then wash the plates 3× with PBS and store at –20°C until required. 3. Incubate T cells at 1 × 106/mL with OKT3 (100 ng/mL) for 1 h at 4°C. 4. Wash once in PBS, resuspend in CCM at 5 × 105/mL, and dispense 200 µL per GAM-coated well. Culture for 3–4 d.

3.3.2. Plate-bound OKT3 + Fibronectin (FN) 1. Precoat wells with 100 ng/well OKT3 in 50 mM Tris-HCl, pH 9.5, and leave overnight at 4°C. 2. Flick off the supernatant and without washing the plates, add a freshly prepared solution of FN (1 µg/50 µL/well, diluted in PBS). 3. Incubate at room temperature for 2 h, wash 3× in PBS, and store at 4°C until required. 4. Dispense 200 µL of T cells at 5 × 105/mL to each well. Culture for 3–4 d.

3.3.3. Immobilized 64.1 MAb 64.1 is diluted in PBS to 1 mg/mL (aliquot and store at –20°C). 1. Add titrated amounts of 64.1 (1000 ng–20 ng/50 µL/well) to flat-well plates and leave at room temperature for 5 h. 2. Wash 3× in PBS and leave the plates wrapped in alfoil at 4°C (long-term storage at –20°C). 3. Start the experiment by adding 200 µL of T cells at 5 × 105/mL and culture for 3–4 d (see Note 9).

3.3.4. Anti-CD2-Induced T-Cell Activation in the Presence of Cofactors 1. Titrate both T112 and T113 together to determine a concentration that does not induce activation in pure T-cell preparations, that is, a submitogenic dose (in our hands this is usually a 1:1000 or 1:2000 dilution of each ascites fluid). 2. Full activation, including IL-2 secretion, requires a second costimulatory stimulus. To this end, we have used a variety of cofactors including cytokines IL-2 (20 U/mL), IL-1β (20 U/mL), IL-6 (200 U/mL), and IL-7 (100 U/mL); PMA (1 ng/mL); MAb anti-CD44 (1:1000 ascites); and the anti-CD28 MAb 9.3 (1:1000 ascites) (see Note 10). The above concentrations are a guide only.

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3. Dispense the anti-CD2 MAbs and graded amounts of cofactor(s) to microtiter wells, add 2 × 105 T-cells per well, and culture for 3–4 d. Thus the capacity of each cofactor to induce activation is easily tested. Anti-CD28 MAbs are excellent costimulatory reagents, presumably because they mimic the key CD28–CD80 signal transduction pathway.

3.4. RT/PCR Analysis We have established a reliable and semiquantitative RT-PCR technique that is based on taking several 5-µL aliquots during the linear phase of the PCR and relating the amount of target product to two control genes, actin and CD3 δ, for a given starting cell number. However, good commercial kits are now available for quantitative RT-PCR that are designed for a number of cytokines. 1. After 4–48 h of culture, harvest as few as 2 × 105 and up to 1 × 106 stimulated and nonstimulated (control) T cells from 24-well Costar plates and wash twice in PBS-containing 0.01% diethylpyrocarbonate (DEPC) (an RNase inhibitor) (see Note 11). 2. To extract cytoplasmic RNA, pellet the cells in Eppendorf tubes and remove as much PBS as possible. Lyse the cell pellets in 0.1 mL of solution A (10 mM Tris, pH 7.5; 150 mM NaCl; 0.65% NP-40; and 10 mM vanyl ribonucleoside complexes). After about 10 s, centrifuge the tubes at 12,000g for 1 min and transfer the supernatant containing the cytoplasmic fraction (be careful not to disturb the pellet) to a new tube containing 0.3 mL of solution B (10 mM sodium acetate, pH 5.0; 50 mM NaCl; 5 mM EDTA; and 0.5% sodium dodecyl sulfate [SDS]). 3. Vortex-mix the mixture and then extract twice with 800 µL phenol/chloroform (1:1) and finally once more with chloroform alone. 4. Remove about 40 µL of the aqueous phase and use an aliquot for the cDNA reaction. Store the remainder at –80°C as 1250 or 2500 cell equivalents per microliter of supernatant. The RNA is stable for at least 12 mo. 5. First-strand cDNA is synthesized from 5 to 10 µL of RNA supernatant, using M-MLV reverse transcriptase (BRL) and oligo dT priming according to the manufacturer’s directions, in a final volume of 40 µL. The reaction is carried out at 37°C for 1 h. 6. One eighth of the cDNA is then added to 45 µL of PCR mix and the tubes subjected to 27–34 cycles of PCR amplification using a thermal cycler with a 1 min/ 95°C denaturation, 2 min/60°C annealing, and 3 min/72°C extension profile. 7. A 5 µL aliquot is taken at the end of several cycles (e.g., 27, 29, and 31), mixed with loading buffer, and then analyzed by electrophoresis on 2% agarose gels and subsequently stained with ethidium bromide (Et Br). A 123-basepair DNA ladder (Gibco-BRL) is used as a marker. Compare and contrast each gene product with the actin and CD3 δ product. Thus its possible to screen about 30–40 different genes from a single cell preparation. For relevant primer sequences see (1).

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Having identified a candidate gene(s) it is important, however, to examine not just message but where possible, protein levels (ELISA) and functional integrity (bioassay).

3.5. T-Cell Adherence to Endothelial Cells (ECs) This simple assay measures the capacity of resting and activated T-cells to adhere to activated or resting endothelium (see Note 12).

3.5.1. Preparation of Human EC 1. Human ECs are prepared by washing intact human umbilical veins, one end closed with a sterile stopcock and valve, with 20 mL of Hanks’ balanced salt solution (HBSS) containing 0.08% collagenase. 2. After 10 min at 37°C, 20 mL of medium 199 is injected into the vein and the collagenase solution washed out. The vein is massaged with fresh medium and the stripped EC collected by centrifugation. 3. ECs are cultured in medium 199 supplemented with 20% FCS and 20 µg/mL Endothelial Cell Growth Factor Supplement (Integrated Sciences) in flasks (Costar) precoated with 2.5 mL of gelatin (a 2% solution in HBSS). 4. To remove the ECs from a flask, treat with 5 mL of trypsin/EDTA solution for 5 min, then wash twice and resuspend the cells in 3 mL of the above medium. 5. Add 1 mL per well to a 24-well culture plate containing a sterile round glass coverslip (sterilized by soaking in ethanol, flaming, and washing in sterile PBS). After 2 d the cells form a new monolayer. 6. Alternatively, monolayers can be derived from the spontaneously transformed immortal human endothelial cell line, namely ECV304, by culturing cells at 2 × 105/mL in 24-well plates in medium 199 plus 10% FCS. 7. To prepare activated EC monolayers, incubate with 5 ng/mL of tumor necrosis factor-α (TNF-α) for 4 h and then wash the cells three times with PBS.

3.5.2 Adherence Assay 1. Dispense half a million resting or activated pan T cells (e.g., activation can achieved by incubating the cells with plate-bound MAb 64.1 for 4 h) to the EC-coated glass coverslips. 2. After 90 min at 37°C wash the coverslips twice in PBS and incubate with 200 µL of MAb; for example, anti-CD3, anti-CD4 (helper T cells), anti-CD8 (supressor/ cytotoxic T cells), anti-CD45RA (naive T cells), anti-CD45RO (memory T cells), or irrelevant MAbs (negative controls). 3. Incubate for 20 min at 4°C and then wash the coverslips twice with PBS. 4. Add 200 µL of pretitrated FITC-labeled goat anti-mouse IgG per well for 20 min at 4°C. 5. Wash the coverslips thrice and remove them from the wells using a needle with a bent tip and place on a glass slide for subsequent examination by UV microscopy. T-cells bound to the EC monolayers are easily identified by their staining pattern with the appropriate MAb.

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4. Notes 1. Test several different FCS batches and select one that is neither inhibitory in the presence of known activators nor mitogenic in the absence of activators. It should also be relatively cheap and available in sufficient quantity to complete all experiments. FCS can be replaced with pooled human AB sera, however, varying amounts of platelet-derived growth factor (PDGF) in human sera may alter T-cell activity, particularly cytokine production. “Old” T-cells are reported to be sensitive to PDGF. If possible, therefore, use a proven serum-free medium or supplement, such as Stratagenes Cell/Perfect PBL serum-free media supplement, that consistently supports T-cell proliferation in your hands. 2. I recommend flat 96-well microtiter plates (Linbro) or 24-well plates (Costar) with lids but different types and brands should be compared. Some plates are clearly better at supporting cellular proliferation than others. (Note that roundbottom plates are optimal for MLR). 3. Identify the optimal and suboptimal concentrations of all exogenous stimuli by running dose–response curves and kinetic studies over several days. The importance of these experiments cannot be overstated, and they should be performed on cells from both aged and young control subjects. Such experiments also help to define the experimental conditions, and indeed help exploit the differences that may exist between age groups. For example, “old” T-cells that do not respond to a particular stimulus at one concentration may respond well at a higher concentration or a day later, when compared to “young” cells. 4. Determine cell viabilities (should be >95%). If